Dissociate LanguagePack and Service objects

To implements bp/store-language-packs-in-glance LP first need to be
standalone objects and not be tied to service.

This patch removes the link between a LanguagePack and its service,
making things much simpler.

It's a first start toward having LP fully into glance

Fixes bug 1301426

Change-Id: Ia7920d288a99dfda8f22a8e987d2ac53197b7e2a
Merge "Dissociate LanguagePack and Service objects"