qa/tasks/cephadm: conditionally pull cephadm binary from chacra

qa/tasks/cephadm: conditionally pull cephadm binary from chacra

When a specific (older) version of cephadm is needed for a test, pull
it from chacra. If we need to bootstrap an older version of ceph, we
pull an older "single-script" binary using the older method.

Co-authored-by: Michael Fritch <mfritch@suse.com>
Co-authored-by: John Mulligan <jmulligan@redhat.com>
Signed-off-by: Michael Fritch <mfritch@suse.com>
Signed-off-by: John Mulligan <jmulligan@redhat.com>