docker_mock: fix remember_tags functionality
docker_mock: fix remember_tags functionality

Previously tagging an image without an explicit tag would
trick this stub into thinking the image was not present,
as inspection always uses an explicit tag.

jarodwilson/atomic-reactor,vrutkovs/atomic-reactor,maxamillion/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor,jarodwilson/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,vrutkovs/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor