Minor fixes for container image prepare command
Minor fixes for container image prepare command

This makes two changes:
- makes sure the documented command written to the env file is valid
  by prepending "openstack"
- removes the "params" entries from the container_images list
  regardless of whether --env-file is specified

Change-Id: I3343df54741e13ddd5a4aa991a5af68c0593833c

openstack/python-tripleoclient