Add image_id method and update openstack_command

This change adds a new method to libraries/cli.rb which we will use
in new cookbook-openstack-integration-test to obtain a glance image id
from a glance image name.

Additionally, we've had to update openstack-command method to handle
situation where we have commands like "glance image-show <id|name>".
Currently, it only seems to work with commands like
"keystone user-list".

Change-Id: Ie4b063340e813ac50cbd4b21e41cc3fa65eebeca
Merge "Add image_id method and update openstack_command"