UCA: Uncomment UCA variables in defaults, fix consequent breakage

UCA: Uncomment UCA variables in defaults, fix consequent breakage

The Ubuntu Cloud Archive-related (UCA) defaults in
roles/ceph-defaults/defaults/main.yml were commented out, which means
if you set `ceph_repository` to "uca", you get undefined variable
errors, e.g.

```
The task includes an option with an undefined variable. The error was: 'ceph_stable_repo_uca' is undefined

The error appears to have been in '/nfs/users/nfs_m/mv3/software/ceph-ansible/roles/ceph-common/tasks/installs/debian_uca_repository.yml': line 6, column 3, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

- name: add ubuntu cloud archive repository
  ^ here

```

Unfortunately, uncommenting these results in some other breakage,
because further roles were written that use the fact of
`ceph_stable_release_uca` being defined as a proxy for "we're using
UCA", so try and install packages from the bionic-updates/queens
release, for example, which doesn't work. So there are a few `apt` tasks
that need modifying to not use `ceph_stable_release_uca` unless
`ceph_origin` is `repository` and `ceph_repository` is `uca`.

Closes: #3475
Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>
(cherry picked from commit 9dd913cf8a3dcc12683b55ae13d95bca6f15cd32)