Periodical backend availability check

[mohierf]

When the backend is not available, this exception on start should be catched and the module should wait for backend availability:

[1447741566] ERROR: [broker-master] The module alignakbackendbrok raised an exception Backend error code 1000: Backend exception: <class 'requests.exceptions.ConnectionError'> / ('Connection aborted.', error(111, 'Connection refused')), I remove it! traceback=Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/alignak/modulesmanager.py", line 211, in get_instances
    inst = module.get_instance(mod_conf)
  File "/usr/local/lib/python2.7/dist-packages/alignak_module_backend/broker/module.py", line 51, in get_instance
    instance = AlignakBackendBrok(mod_conf)
  File "/usr/local/lib/python2.7/dist-packages/alignak_module_backend/broker/module.py", line 66, in __init__
    getattr(modconf, 'allowgeneratetoken', False))
  File "/usr/local/lib/python2.7/dist-packages/alignak_module_backend/broker/module.py", line 93, in getToken
    self.backend.login(username, password, generate)
  File "/usr/local/lib/python2.7/dist-packages/alignak_backend_client/client.py", line 138, in login
    raise BackendException(1000, "Backend exception: %s / %s" % (type(e), str(e)))
BackendException: Backend error code 1000: Backend exception: <class 'requests.exceptions.ConnectionError'> / ('Connection aborted.', error(111, 'Connection refused')

[mohierf]

Exception is catched but a periodical check to test backend availability is not yet implemented nor tested. Change issue name to reflect this

[mohierf]

Fixed, but currently the missed broks are lost ... we should implement a local cache

[mohierf]

A periodical backend check is implemented in the logs and ws modules ... amy be we should backport this feature in the backend modules ?