test/rgw: Cluster.admin() returns output as utf-8 str

test/rgw: Cluster.admin() returns output as utf-8 str

fixes the TypeError:

  File "rgw_multi/tests.py", line 1107, in test_bucket_sync_disable
    check_buckets_sync_status_obj_not_exist(zone, buckets)
  File "rgw_multi/tests.py", line 438, in check_buckets_sync_status_obj_not_exist
    if log_list.find(':'+bucket+":") >= 0:
TypeError: argument should be integer or bytes-like object, not 'str'

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit ff5ce2cf1457e1945aea0fe4a023bc48bffe1174)