Setup loop devices early on in the Ceph tests

There appears to be a race condition between the loop device creation
and its actual usage. In order to prevent that, this PR setups these
devices during the test initialization instead of right before using
them in the 'add-disk' tests.

zaza/openstack/charm_tests/ceph/tests.py

@@ -186,6 +186,11 @@ class CephTest(test_utils.OpenStackBaseTest):
     def setUpClass(cls):
         """Run the ceph's common class setup."""
         super(CephTest, cls).setUpClass()
+        cls.loop_devs = {}   # Maps osd -> loop device
+        for osd in (x.entity_id for x in zaza_model.get_units('ceph-osd')):
+            zaza_model.add_storage(osd, 'cache-devices', 'cinder', 10)
+            loop_dev = zaza_utils.add_loop_device(osd, 10).get('Stdout')
+            cls.loop_devs[osd] = loop_dev
 
     def osd_out_in(self, services):
         """Run OSD out and OSD in tests.
@@ -565,8 +570,7 @@ class CephTest(test_utils.OpenStackBaseTest):
         osds = [x.entity_id for x in zaza_model.get_units('ceph-osd')]
         params = []
         for unit in osds:
-            zaza_model.add_storage(unit, 'cache-devices', 'cinder', 10)
-            loop_dev = zaza_utils.add_loop_device(unit, 10).get('Stdout')
+            loop_dev = self.loop_devs[unit]
             params.append({'unit': unit, 'device': loop_dev})
             action_obj = zaza_model.run_action(
                 unit_name=unit,