[system tests] KeyError 'mons' in tests with ceph

On tests with ceph we got key error:
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 331, in run
    testMethod()
  File "/usr/lib/python2.7/unittest/case.py", line 1043, in runTest
    self._testFunc()
  File "/home/jenkins/venv-nailgun-tests-2.9/local/lib/python2.7/site-packages/proboscis/case.py", line 296, in testng_method_mistake_capture_func
    compatability.capture_type_error(s_func)
  File "/home/jenkins/venv-nailgun-tests-2.9/local/lib/python2.7/site-packages/proboscis/compatability/exceptions_2_6.py", line 27, in capture_type_error
    func()
  File "/home/jenkins/venv-nailgun-tests-2.9/local/lib/python2.7/site-packages/proboscis/case.py", line 350, in func
    func(test_case.state.get_state())
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/helpers/decorators.py", line 70, in wrapper
    result = func(*args, **kwargs)
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/tests/test_ceph.py", line 751, in check_ceph_partitions_after_reboot
    self.fuel_web.check_ceph_status(cluster_id)
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/__init__.py", line 48, in wrapped
    result = func(*args, **kwargs)
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/models/fuel_web_client.py", line 1732, in check_ceph_status
    self.check_ceph_time_skew(cluster_id, offline_nodes)
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/helpers/decorators.py", line 331, in wrapper
    return func(*args, **kwargs)
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/models/fuel_web_client.py", line 1677, in check_ceph_time_skew
    skewed = ceph.get_node_fqdns_w_time_skew(remote)
  File "/home/jenkins/workspace/6.1.system_test.centos.ceph_ha_one_controller/fuelweb_test/helpers/ceph.py", line 102, in get_node_fqdns_w_time_skew
    monitors = health['timechecks']['mons']
KeyError: 'mons'

https://github.com/stackforge/fuel-qa/blob/master/fuelweb_test/helpers/ceph.py#L102