Fix Ubuntu policy class

Make sure the ubuntu policy class imports the DebianPlugin tagging
class and move initialisation off valid_subclasses from class
scope to the constructor since the superclass will reset the
property value when its initialiser is called.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Merge branch 'master' into fix-tabs-to-spaces-in-plugins