MB-100: handle ServerUnavailableException in rebalance progress

Otherwise rebalance monitor fails sometimes:

File "lib/membase/api/rest_client.py", line 863, in monitorRebalance
  progress = self._rebalance_progress()
File "lib/membase/api/rest_client.py", line 909, in _rebalance_progress
  status, content, header = self._http_request(api)
File "lib/membase/api/rest_client.py", line 610, in _http_request
  raise ServerUnavailableException(ip=self.ip)
ServerUnavailableException: unable to reach the host @ 172.23.96.15

Change-Id: I8fa12774f5469357372a56740ccab58603877356
Reviewed-on: http://review.couchbase.org/25466
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>