Remove uses of instance['instance_type'] from nova/notifications

This is one change in a series aimed at removing the use of instance-linked
instance_type objects, in favor of the decoupled type data in
system_metadata. See bug 1140119 for more details.

Change-Id: Iebbb0fa0bf306d6a7c8b83594e13a4b99f27c404
Merge remote-tracking branch 'm_src_repo/master' into build-temp