public_endpoint & admin_endpoint configuration

Today we can use these configuration values to avoid having to guess
keystone's own endpoint URL from the service catalog backend, which may
contain more than one identity endpoint.

This is also the first step towards adding self-relational links to the v3 API.

Change-Id: I375ac0d1f9581592e437c67c17bf32022f652f66
Merge "public_endpoint & admin_endpoint configuration"