Fix exception handling in novaclient

novaclient.exceptions wasn't being imported so was causing a 500
to be thrown if nova raised an exception.

Also rename the novaclient variable to nova to avoid mirroring
an import name.

Change-Id: Ic699c5231b74641bcd8baec4a5761a3fcae726f1

novajoin/nova.py

@@ -15,6 +15,7 @@
 """Handle communication with Nova."""
 
 from novaclient import client
+from novaclient import exceptions
 from oslo_config import cfg
 from oslo_log import log as logging
 
@@ -44,8 +45,8 @@ class NovaClient(object):
 
 
 def get_instance(instance_id):
-    novaclient = NovaClient()
+    client = NovaClient()
     try:
-        return novaclient.client.servers.get(instance_id)
-    except novaclient.exceptions.NotFound:
+        return client.client.servers.get(instance_id)
+    except exceptions.NotFound:
         return None