test/rgw: fix test_encrypted_object_sync for 3+ zones

[cbodley]

new test is failing in the multisite suite with three-zone.yaml because I can't write python:

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ERROR: tasks.rgw_multi.tests.test_encrypted_object_sync
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2017-08-30T03:28:21.255 INFO:tasks.rgw_multisite_tests:Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/virtualenv/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/teuthworker/src/github.com_ceph_ceph-c_wip-yuri-testing4_2017_8_30/qa/tasks/rgw_multi/tests.py", line 938, in test_encrypted_object_sync
    (zone1, zone2,) = zonegroup_conns.rw_zones
ValueError: too many values to unpack

---------------------------------------------------------------------

[theanalyst]

you can drop the tuple syntax too, zone1, zone2 = ... will work (but either ways are fine)

[theanalyst]

seeing this in luminous branch as well, lets get this in & backport to L