Refactor compute manager _get_instances_by_driver

Make the method more efficient by only querying nova-conductor
once. Also limit fallback instance query to host only. This is
in preparation for fixing bug 1129519.

Change-Id: I7de14f1ffda37c14f760f602fd81250d2163b430
Merge "Refactor compute manager _get_instances_by_driver"
Merge remote-tracking branch 'm_src_repo/master' into build-temp