qa/cephadm: use squid image as default for test_cephadm workunit

qa/cephadm: use squid image as default for test_cephadm workunit

This patch is intended directly for the squid branch. On squid,
we should be running tests using the squid image. This should have
been updated at some point after squid branched off from main. This
hasn't started causing any actual test failures yet, but will as
soon as the version on main goes from 19 -> 20

Fixes: https://tracker.ceph.com/issues/64208
Signed-off-by: Adam King <adking@redhat.com>