Add API discovery and commands

* add low-level API discovery support
* add 'api list' and 'api match' commands

This is incomplete WRT other APIs, only Identity is discovered at the moment.

TODO: Fix an issue mocking the ClientManager tests with the addition of
discovery...

Change-Id: I3cdce4b2b6057efa2f517ac1e00bc8c266dcb1db
Merge branch 'oslo-clean' into dt-test
Merge branch 'image-v2-v2' into dt-test
Merge branch 'identity-ec2-tests' into dt-test