Cinder get-capabilities tests

Cinder allows getting capabilities for a storage back end

Added:
- Api for get-capabilities
- Documentation reference for cinder show_pools

Tests:
- Test get capabilities for a given back end
- Test get volume stats
Note: Only admin has the privileges to perform get-capabilities action,
by default.

Change-Id: I1329a1339e1d8526f94ee57fb0ab43a78d881174
2 files changed