contrib: stop building and pushing ubuntu on mimic
contrib: stop building and pushing ubuntu on mimic

We don't always have packages for ubuntu and we can't hold a release
just because of that. So until we get a proper maintainer for the
ubuntu image we will stop building and pushing the image. However it's
up to anyone to build it locally.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/ceph-docker,ceph/ceph-docker