channel_config module API modifications

[alxiong]

This issue contains several discussions over the current channel_config API

• the logic of checking a channel already exists should be included in the create channel_config task -- leaving this to be manually specified by users are slightly troublesome and prone to errors.
  So the return value might require changes:
  path shouldn't be returned since it is required as an input parameter;
  exist: bool could be added as the return of create action

• AFAIU, create contains both "creating a new channel" AND "updating an existing channel". If so, maybe this should be noted in the documentation, as the task is being named as "creating a new channel config"

• fetch operation should be of a separate module called channel_config_info which follows the commonly shared pattern of other modules (certificate_authority and certificate_authority_info etc.)
  The primary reason for this separation is cleaner, consistent API, since fetch would return a channel_config object whereas create, update, sign and update might at most returns a status bit.

• in the doc, *-policy.json is used whereas in tutorial/08-*.yml, *.json.j2 is used. Might be great to document down the various format supported and give example of commonly used policies including m-out-of-n channel update policy, operator, writer, reader permission etc.

To summarize, here is a tentative todo list:

• merge channel existence check into create API logic
• revise on return API of channel_config module
• move fetch into a separate module named channel_config_info
• update documentations on "create & update channel config", and on "policy examples & supported format"

👍 thanks! All suggestions above would be subjected to further discussions. Please let me know your thought~

[sstone1]

Haven't forgotten about this one!

FYI, operation: create only allows you to create new channels, it does not allow you to update them.

I'm not sure about the splitting out of operation: fetch into a channel_config_info. The _info modules generally return information as the result, and are read-only. However operation: fetch fetches the latest config and writes it to the file system, and does not return any useful information in the result.