Fix list_containers() when there are no containers

nova-compute won't start if there are no containers running
as "docker ps -a" returns a 404 and not a json blob as it expects.
This patch just checks for a 404 and returns an empty list.

Change-Id: I1b02951801808eb19d76d85dcd5245cf7d9a25f1
Merge "Fix list_containers() when there are no containers"