Don't fall when just loading designatev2 modules

this avoids loading warnings on salt-master that may not have
os-client-config installed, as salt tries to load them irrespective of
__virtual__() result.

Change-Id: Ib95887c274179130f87d4897e55779bc1c4f6206
Related-Issue: https://mirantis.jira.com/browse/PROD-26920
diff --git a/_modules/designatev2/common.py b/_modules/designatev2/common.py
index 22e8ad3..1bee497 100644
--- a/_modules/designatev2/common.py
+++ b/_modules/designatev2/common.py
@@ -1,5 +1,9 @@
 import logging
-import os_client_config
+try:
+    import os_client_config
+except ImportError:
+    os_client_config = None
+from salt import exceptions
 
 log = logging.getLogger(__name__)
 
@@ -42,6 +46,10 @@
 
 
 def _get_raw_client(cloud_name):
+    if not os_client_config:
+        raise exceptions.SaltInvocationError(
+            "Cannot load os-client-config. Please check your environment "
+            "configuration.")
     service_type = 'dns'
     config = os_client_config.OpenStackConfig()
     cloud = config.get_one_cloud(cloud_name)