Functionality for rolling a policy back to a previous revision

[dseevr]

Since we now record a revision entry for each policy upload, we should also provide a way to roll a policy back to a previous revision.

This could be a volcli command like: volcli policy rollback policy_name target_revision

Internally, the volcli command would need to:

• ensure the target revision exists in the policy's revision history
• ensure the target revision is not the current revision (if this is true, just print a notice and exit)
• fetch the policy JSON and reupload it through the normal apimaster endpoint