[Katello] Including the Engine in the subject

I'd like to request that users of this mailing list (especially Katello
devs) adopt a policy of including [Katello] in the subject to help denote
Katello related posts and help with filtering. Big ups to Justin for
starting this trend on the mailing list.

This may also be helpful, if not already being done, for discussions
targeted at other engines as well.