Use idiomatic style

Switch from long check to catching exception to make code more idomatic.

charms_openstack/adapters.py

@@ -158,12 +158,14 @@ class PeerHARelationAdapter(OpenStackRelationAdapter):
                         'network': 'this_unit_private_addr/private_netmask'}}
         """
         relation_info = {}
-        cluster_relids = hookenv.relation_ids('cluster')
-        if (cluster_relids and not
-                hookenv.related_units(relid=cluster_relids[0])):
-            relation_info = {
-                'cluster_hosts': self.local_default_addresses(),
-            }
+        try:
+            cluster_relid = hookenv.relation_ids('cluster')[0]
+            if not hookenv.related_units(relid=cluster_relid):
+                relation_info = {
+                    'cluster_hosts': self.local_default_addresses(),
+                }
+        except IndexError:
+            pass
         return relation_info
 
     def local_network_split_addresses(self):