docker-puppet.py: don't pull if image exists
docker-puppet.py: don't pull if image exists

This updates the pull function here so that it matches
how paunch also handle's images, only pulling them if
the existing image doesn't already exist on the host.

Change-Id: I90ea41ccdfdb0b9206a63901554d002a5ec0fd3a

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates