Received notifications context is not a DesignateContext

Nova/Neutron's notifcations context contains fields which we do not
know how to handle in DesignateContext. We should treat received
context's as simple dict's rather than attempting to munge into
a DesignateContext.

Change-Id: Ieda187679171333be69e98d6301c64a14187a5d7
Closes-Bug: 1360696
Merge "Received notifications context is not a DesignateContext"