Consume packages from download.ceph.com for ceph octopus
Our focal fossa based CI jobs are getting octopus from
ubuntu ppas instead of getting the packages from
download.ceph.com.
This happens because, even though we were defaulting to
nautilus, ubuntu got the later version available
(in this case, the packages in ubuntu ppas).
Also, remove CEPH_STABLE_RELEASE env var, unify to CEPH_RELEASE
Stable branches CI versioning is done in the jobs definition now
Change-Id: I1edd5a50079f325fa143a7e0d51b3aa3bb5ed45d