Retrieve missing container command after pulling from index

Docker driver spawns container with wrong command "sh"
if image doesn't exists on nova node.
'Cmd' assigned before pulling image, hence docker 'inspect'
of unexisting image returns empty command.
Solution: refactor code to first check if image exists,
pull if not and inspect only after pulling.

Change-Id: Id30d20bb57946ab11f67c96ffbb5907006b6d558
Closes-Bug: #1266875

Conflicts:
	nova/tests/virt/docker/test_driver.py
	nova/tests/virt/test_virt_drivers.py