Add basic Keystone CLI tests

New cli.output_parser helpers for parsing 'ascii-tables'
 often used in CLIs output.

New basic tests for keystone cli, verifies only read-only
 actions (return codes and basic structure of output).

Change-Id: I4fea08b14e32c62c47e347b401e3f5703836c184
Merge "Add basic Keystone CLI tests"