Always pass context to compute driver destroy()

The context arg was added to the compute driver's destroy() method in
commit 8c0673d3102478ef34ef1b11c1db156db4b68f62.  The context is used by
the libvirt driver when dealing with encrypted volumes.

Unfortunately, most of the places where this method is called in the
compute manager, the context wasn't being passed in.  This patch makes
the argument mandatory and updates all of the drivers to match.

Change-Id: Icea2a36c16869af43954065473ff4e4910f32b1e
Merge "Always pass context to compute driver destroy()"