Update cloud/docker: if the image name containes a repository, strip it.

When using repositories other than the main one at docker.io, the image name contains the repo name (which itself contains ":" as a separator between domain and port). We don't really care about it here, so just get rid of it before looking at the image name.

This is a backport from https://github.com/cove/docker-ansible/ as per @cove's request in https://github.com/cove/docker-ansible/pull/19#issuecomment-28628426