Fix ec2 credentials commands for new auth

These commands were not updated for the new authentication model.  Also
add functional tests for 'ec2 credentials' commands.

Also fix tenant_id in output for create and show.

Closes-Bug: 1399757

Change-Id: I5d4beb9d1fa6914fef5e4c7b459cdd967e614b24
Merge branch 'rst-command-list-0' into dt-test
Merge branch 'low-level-api-network' into dt-test