The previous commits adds support for a configurable notification driver
and an mqtt publisher implementation. The subscriber interface was left
hard coded to the pika implementation and respecting the configurable
driver config setting was left as a future item. This commit addresses
this and adds the missing subscriber interface for the configurable
notification drivers with an mqtt implementation.
Change-Id: I97aafc5fd94ac1a8c126fa1f463e33e772bf87f0