rgw: ignore zoneless default realm when not configured

rgw: ignore zoneless default realm when not configured

"default" zone/zonegroup deployments without a realm can be broken by
the creation of an unrelated realm, because that realm is (was)
automatically set as the default

when startup detects an incomplete default realm (one that doesn't have
a default zone), fall back to the realmless "default" zone/zonegroup
instead

Fixes: https://tracker.ceph.com/issues/67697
Signed-off-by: Casey Bodley <cbodley@redhat.com>