Bugzilla plugin is now converted to backend sync service

as the summary, Bugzilla is now a backend sync service. It can be used simply as it was a plugin before. When enable this service, any added task will be handled and attempt to sync bug information from specific related Bugzilla service.

This version works well, but not perfect. I'm being continue to add more features, such as the configuration ability.
This was wrote a bit long time ago in the bzr time. Tried to recall the reason. Major one is bugzilla.samba.org fails to response request. Server returns "mismatched tag: line 98, column 4".

This is a flag for admin purpose internally to disable current invalid bz service. GTG has a long history of developement and many users are using it. I don't know and quite sure how the original bugzilla plugin affects users. So, using this flat to disable it temporarily.

As mentioned above, I'm gonna give user the ability to configure bz services in the backend dialog. This flag will be necessary to exist and be removed in turn.