Fix VM issues.

Fix of #5494 although bad description. Instead of adding a wait
the code used to detect if the guest was back up is fixed. The
previous code appeared to assume only one machine and broke
when it was waiting for multiple machines if the guests did not
come up within 10 seconds of each other

Make nuke not do the normal stuff if the machine is a VPS as we
just destroy them when they get unlocked.

Instead of getting downburst options from ~/.teuthology.yaml get
it from the yaml given to teuthology for the test/task instead.

Fixed an error that would make all the default downburst values
not take effect if any of them were set via a yaml.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>