qa/cephadm: derive container image from cephadm release

qa/cephadm: derive container image from cephadm release

test_cephadm.sh hardcodes IMAGE_DEFAULT to ceph:main, which breaks
every stable branch whenever main is renamed to a new release. The
mismatch check in cephadm correctly rejects the container because its
release name doesn't match cephadm's own release. This has recurred on
every release transition (squid→tentacle, quincy→reef) without a fix.

Instead of always pulling ceph:main, derive IMAGE_DEFAULT from the
installed cephadm's version output. On stable builds (release type
"stable"), use ceph:<release> so the container matches cephadm. On dev
builds (main branch), fall back to ceph:main as before. The IMAGE_DEFAULT
env var can still be set externally to override.

Fixes: https://tracker.ceph.com/issues/75821
Signed-off-by: Lumir Sliva <61183145+lumir-sliva@users.noreply.github.com>