Add LVM as default backend

This change set the enable-backends cinder config option to the
package default of 'lvm'. The reason is to stop the service flapping
and fix the bug which is causing cinder-volume service to be down
in some deployments.

Change-Id: I29b9bccf75019b73f60c1160b8b6c7cb11d3ad28
Closes-Bug: 1719742

.gitignore

@@ -5,3 +5,4 @@ bin
 *.sw[nop]
 *.pyc
 .unit-state.db
+.stestr

hooks/cinder_contexts.py

@@ -137,12 +137,12 @@ class StorageBackendContext(OSContextGenerator):
                 backends.append('CEPH')
             if enable_lvm():
                 backends.append('LVM')
-        if len(backends) > 0:
-            return {
-                'active_backends': backends,
-                'backends': ",".join(backends)}
-        else:
-            return {}
+        # Use the package default backend to stop the service flapping.
+        if not backends:
+            backends = ['LVM']
+        return {
+            'active_backends': backends,
+            'backends': ",".join(backends)}
 
 
 class LoggingConfigContext(OSContextGenerator):

unit_tests/test_cinder_contexts.py

@@ -113,7 +113,9 @@ class TestCinderContext(CharmTestCase):
         self.config.return_value = None
         self.relation_ids.return_value = []
         self.os_release.return_value = 'havana'
-        self.assertEqual(contexts.StorageBackendContext()(), {})
+        self.assertEqual(
+            contexts.StorageBackendContext()(),
+            {'active_backends': ['LVM'], 'backends': 'LVM'})
 
     def test_storage_backend_single_backend(self):
         rel_dict = {