Fix the shibboleth portion of installation

[tcmitchell]

1. Don't distribute templates/shibboleth2.xml file. That approach is brittle because deployers will have various versions of Shibboleth service provider (SP) installed. Instead, provide some instructions, either general or specific, for configuring the Shibboleth SP. There are only a handful of things that need to be edited, and it can be done manually.
2. Don't distribute the templates/idp-metadata-extension.xml file. MDUI data is the responsibility of the identity provider (IdP), not the SP.
3. Update INSTALL-centos.md accordingly, particularly section 6, "Set up SP with IDP"