rgw: use tenant/ for bucket instance metadata keys

rgw: use tenant/ for bucket instance metadata keys

to work around the ambiguity of parsing tenant: and :shard in the same
bucket instance metadata key, use tenant/ instead

to preserve backward compatibility with existing objects, new helper
function rgw_bucket_instance_key_to_oid() converts this / back to a :
before being used as an object name

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit a0befa1e7be3aeb37209bac2e2a7de0edf5d5a95)