Configure control_exchange to match keystone

keystone is using 'keystone' as default control_exchange [1], barbican needs
to match the value so it could consume related events notifications.

[1] https://github.com/openstack/keystone/blob/master/keystone/conf/__init__.py#L90

Change-Id: I16c0fb7e79545268b058eb6fd1a006a04e3fb61b

barbican/common/config.py

@@ -168,7 +168,7 @@ ks_queue_opts = [
     cfg.BoolOpt('enable', default=False,
                 help=u._('True enables keystone notification listener '
                          ' functionality.')),
-    cfg.StrOpt('control_exchange', default='openstack',
+    cfg.StrOpt('control_exchange', default='keystone',
                help=u._('The default exchange under which topics are scoped. '
                         'May be overridden by an exchange name specified in '
                         'the transport_url option.')),

devstack/lib/barbican

@@ -156,6 +156,7 @@ function configure_barbican {
 
     # Enable the keystone listener
     iniset $BARBICAN_CONF keystone_notifications enable True
+    iniset $BARBICAN_CONF keystone_notifications control_exchange 'keystone'
 }
 
 # init_barbican - Initialize etc.

releasenotes/notes/change_default_control_exchange-c47abc3e3f08aa31.yaml

@@ -0,0 +1,4 @@
+---
+other:
+  - default value of 'control_exchange' in 'barbican.conf'
+    has been changed to 'keystone'.