VMware: ensure rescue instance is deleted when instance is deleted

If the user creates a rescue instance and then proceeded to delete
the original instance then the rescue instance would still be up
and running on the backend.

This patch ensures that the rescue instance is cleaned up if
necessary.

The vmops unrescue method has a new parameter indicating if
the original VM should be powered on.

Change-Id: I3c1d0b1d003392b306094b80ea1ac99377441fbf
Closes-bug: 1269418
Merge "VMware: ensure rescue instance is deleted when instance is deleted"