dockertest.containers: Add list_container_ids()

*  Mirrors similar method in images module,
   splats back a list of just long-ids

*  Added unittest

Signed-off-by: Chris Evich <cevich@redhat.com>
subtests...events: Add events test

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #48 from cevich/docker_events

Docker events