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.
Changed a few things form Josh D's suggestions.

Now using --machine-type instead of --type and changed some oarts
of the code per Josh D's appreciated suggestions =)
Merge branch 'wip-machinetype' of github.com:ceph/teuthology into wip-machinetype