qa/cephadm: add default container image name base

qa/cephadm: add default container image name base

Recent runs in the new lab for cephadm jobs are hitting

```
2026-01-06T00:47:00.247 ERROR:teuthology.run_tasks:Saw exception from tasks.
Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_teuthology_8707f5d0ca0e547efc56a3734c7aa4a4cf45b1f4/teuthology/run_tasks.py", line 112, in run_tasks
    manager.__enter__()
  File "/usr/lib/python3.12/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "/home/teuthworker/src/github.com_ceph_ceph-c_73712ccf0d7b99648a0c0d2ebf766adc13587a45/qa/tasks/cephadm.py", line 1961, in task
    raise Exception("Configuration error occurred. "
Exception: Configuration error occurred. The 'image' value is undefined for 'cephadm' task. Please provide...
```

but it appears the build is still pushing images to quay.ceph.io/ceph-ci/ceph:<sha1>
which is the image we would be looking for. The bit that raises an exception if
this isn't set is quite old, and I'm unsure if we still need it or it's safe
to just assume this naming scheme can be used if an explicit image is
passed, but this seems worth a try

Signed-off-by: Adam King <adking@redhat.com>