Rename roles' names

I added a 'ceph-' prefix to all the roles related to Ceph. Since we are
about to push the roles into the Ansible Galaxy that will be easier when
we want to use these roles into a larger environement with other roles.

Fixes: #94

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #95 from leseb/wip-rename-common

Rename roles' names