Short circuit notifications when not enabled

Wasting time on generating the payload of the notifications is
unnecessary if the notification will not be emitted due to
configuration.

This patch depends on an oslo messaging improvement that adds
a way to check if a Notifier will emit the notification or not.

Depends-On: Ib992f5c20fef85224fb00823e1d8d9c6cff19bec
Depends-On: I979fb3385671aba6f4162ef991da8f0febe0068a

Change-Id: I3e6741d59df49e1e58409314008c2ed609fdedc1

nova/rpc.py

@@ -215,6 +215,19 @@ def get_versioned_notifier(publisher_id):
     return NOTIFIER.prepare(publisher_id=publisher_id)
 
 
+def if_notifications_enabled(f):
+    """Calls decorated method only if versioned notifications are enabled."""
+    @functools.wraps(f)
+    def wrapped(*args, **kwargs):
+        if (NOTIFIER.is_enabled() and
+                CONF.notifications.notification_format in ('both',
+                                                           'versioned')):
+            return f(*args, **kwargs)
+        else:
+            return None
+    return wrapped
+
+
 def create_transport(url):
     exmods = get_allowed_exmods()
     return messaging.get_transport(CONF,