refactoring ubuntu/debian change_host_name

there's been a lot of churn around this code, so i figure it was worth
trying to clean it up.

 - the methods were doing a lot, so make them into template methods with
   one helper per step
 - spread out /etc/hosts regexp into a couple of helper variables for
   clarity
 - remove handling for broken hostname implementations (like basing all
   of the checks on name.split('.')[0]), since it seems reasonable to
   remove code dedicated only to handling broken boxes
 - DRY up the shared code between debian/ubuntu implementations, which
   clarifies the differences as well
 - add unit tests around the behavior; this will help us in the future
   to separate flaws in our understanding from flaws in implementation
 - includes a new DummyCommunicator in tests which should be useful in
   supporting additional unit testing of this kind
 - manually tested this on squeeze, wheezy, precise, quantal, raring,
   and saucy successfully.

handles the issue in #2333
Merge pull request #2534 from phinze/2333-debian-ubuntu-change-host-name-refactor

guests/debian,ubuntu: refactoring ubuntu/debian change_host_name