Plugin refactoring plan

First iteration of the plan to split ocp-velero-plugin into
two separate velero plugins:

1) A general openshift plugin, appropriate for situations
   in which velero is used for general backup and restore
   of OpenShift namespaces.
2) A migration plugin, intended to be used only in conjunction
   with the Migration Controller.

docs/plugin-refactoring.md

@@ -0,0 +1,145 @@
+# ocp-velero-plugin Repo Split
+
+Currently, the contents of ocp-velero-plugin includes a mixture of
+actions that are relevant for general backup and restore of OpenShift
+namespaces, and actions that only make sense in the context of a
+controller-run migration. This document is an attempt to separate the
+two purposes in preparation for splitting the repo into two.
+
+## General backup/restore features (and supporting infrastructure):
+
+* Record backup/restore cluster version and registry hostname.
+* Don't restore pods that are controlled by higher-level entities (DeploymentConfig, etc.).
+* Modify image references that point to the internal registry.
+* Modify builds that point to the internal registry and update the builder-dockercfg secret.
+* Modify subdomain on route to match dest cluster.
+
+## Only needed for migration:
+
+* ConfigureContainerSleep and plugins that deal with this
+* ImageStream/ImageStreamTag image copy and "item skipping"
+* PV/PVC actions
+* Quiesce pods on final migration
+
+## Post-split:
+
+The general openshift velero plugin will be called
+openshift-velero-plugin. The migration plugin will be
+openshift-migration-plugin.
+
+Once the repos are split, the migration-specific plugin image will
+include the general plugin code (and register any general plugins that
+will be used unchanged for migration). As a result, a velero
+installation used for migration would only need to have the migration
+plugin added, not the general plugin. Another approach would be to
+keep them separate and require both plugins to be added, but this
+reduces our flexibility to handle situations where we would want a
+migration-specific plugin action registered *instead of* a general
+plugin rather than *in addition to* it.
+
+We should be able to remove the conditional statements that skip
+plugin actions for non-migration use cases, since the migration plugin
+won't be registered for general backup restore. This means that most
+of the code like this won't be necessary anymore:
+```
+	if input.Restore.Annotations[common.MigrateCopyPhaseAnnotation] != "" {
+```
+We will still need conditionals where the action differs based on copy
+vs. final migrations or copy vs. move behavior.
+
+## File-by-file
+
+
+* build
+  * General
+    * restore.go, restore_test.go
+* clients
+  * General
+    * clients.go
+* common:
+  * General
+    * backup.go  
+    * restore.go
+    * shared.go:
+      * func getMetadataAndAnnotations
+      * func GetServerVersion
+      * func GetRegistryInfo
+    * types.go:
+      * type APIServerConfig
+      * const BackupServerVersion
+      * const RestoreServerVersion
+      * const BackupRegistryHostname
+      * const RestoreRegistryHostname
+    * util.go
+      * func ReplaceImageRefPrefix
+      * func HasImageRefPrefix
+      * type LocalImageReference
+      * func ParseLocalImageReference
+      * func SwapContainerIMageRefs
+      * func GetSrcAndDestRegistryInfo
+      * func GetOwnerReferences
+  * Migration-specific:
+      * const MigrationRegistry
+      * const SwingPVAnnotation
+      * const MigrateTypeAnnotation
+      * const MigrateCopyPhaseAnnotation
+      * const MigrateQuiesceAnnotation
+      * const PodStageLabel
+      * const ResticRestoreAnnotationPrefix
+      * const ResticBackupAnnotation
+    * util.go
+      * func ConfigureContainerSleep
+* daemonset:
+  * General
+    * restore.go
+* deployment:
+  * General
+    * restore.go
+  * Migration-specific:
+    * backup.go (quiesce behavior)
+* deploymentconfig:
+  * General
+    * restore.go
+  * Migration-specific:
+    * backup.go (quiesce behavior)
+* imagestream:
+  * Migration-specific:
+    * backup.go, restore.go, shared.go (it's possible that we would want a general plugin
+      for imagestream that's not used for migration, but nothing in these files should be used outside of migration)
+* imagestreamtag:
+  * Migration-specific:
+    * restore.go   (it's possible that we would want a general plugin
+      for imagestream that's not used for migration, but nothing in this file should be used outside of migration)
+* job:
+  * General
+    * restore.go
+* pod:
+  * General
+    * restore.go (swap image refs portion)
+  * Migration-specific:
+    * restore.go (ConfigureContainerSleep portion)
+    * Note: here's an example where we need separate Execute action
+      plugin for general and migration use cases if modifying
+      image references goes into the general plugin
+* pv:
+  * Migration-specific:
+    * backup.go
+    * restore.go
+* pvc:
+  * Migration-specific:
+    * restore.go
+* replicaset:
+  * General
+    * restore.go
+* replicationcontroller:
+  * General
+    * restore.go
+* route:
+  * General
+    * restore.go, restore_test.go
+* statefulset:
+  * General
+    * restore.go
+* util/test:
+  * General:
+    * test.NewLogger