Expose started state of coordinator to external

Users (others projects like Ceilometer) may need to check if the
coordinator has been started or not, but tooz didn't expose an external
interface. This change expose an is_started property that Ceilometer don't
need maintain the state itself any more.

Change-Id: I1a177cf3efd63593667751c6b53bcc536fb0670f

tooz/coordination.py

@@ -172,6 +172,10 @@ class CoordinationDriver(object):
         """
         raise tooz.NotImplemented
 
+    @property
+    def is_started(self):
+        return self._started
+
     def start(self):
         """Start the service engine.
 

tooz/tests/test_coordination.py

@@ -720,6 +720,12 @@ class TestAPI(testscenarios.TestWithScenarios,
         self.assertTrue(lock2.acquire(blocking=True))
         self.assertTrue(lock2.release())
 
+    def test_get_started_status(self):
+        self.assertTrue(self._coord.is_started)
+        self._coord.stop()
+        self.assertFalse(self._coord.is_started)
+        self._coord.start()
+
     @staticmethod
     def _get_random_uuid():
         return str(uuid.uuid4()).encode('ascii')