Add developement packages support

This commits introduces the support of the developpement branches of
Ceph. You can now install Ceph from master.
The behavior is done through 2 new options:

* ceph_stable: true will use the stable branch
* ceph_dev: true will use the dev branch

For the dev packages don't forget to set the branch that you want to
use.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>