Added support for multiple types of machines.

Added the ability to support multiple types of machines with
--machine-type added to teuthology-lock when used with --lock-many
or --machine-type with teuthology --lock (automated tests). It
defaults to 'plana' and the 'vps' type is currently unused but
should be in the future.

Signed-off-by: Sandon Van Ness <sandon@van-ness.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>