Pass an empty context to the notifier

The notifier API expects a context as a first parameter. During the port
to oslo.messaging, a bug was introduced here and the first argument in
the notifier call was removed. This patch puts it back.

Change-Id: Id2f47757ee4af8f8c39a8a1e6b11dcad163f5b7b
Closes-bug: #1346466
Merge "Pass an empty context to the notifier"