Implement full reinstallation of a VM system.

Downburst create is used to reinstall a VM when it is locked.
Downburst destroy is used to remove a VM when it is unlocked.
Host keys are regenerated on each vm instantiation, so the keys
need to be checked prior to use.
If needed, qa-ceph-chef is run on newly installed systems to insure that
they are fully functional.

Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge branch 'wip-teuthologyvminstall-wusui'