Add notification on deleting instance without host

If an instance which does not have a host associated is deleted, the
corresponding notifications were not being sent. Fixed the issue.

Closes-Bug: #1211742

Change-Id: I56482ab38725e6f1ddfb14350637d329f35095b3
(cherry picked from commit bbab55c4da531e4695a6e3e577aaa4975f0fce79)
Merge "Add notification on deleting instance without host" into stable/havana