generate_group_vars_sample: ability to merge roles

generate_group_vars_sample: ability to merge roles

Problem: the meta declaration just includes the role, it does nothing
with the group_vars. For ansible to use files defined in group_vars/ the
name of the file must match a host group. Like mons, osds, etc. There is
no group docker-common so the variables defined there are never used, as
proved by https://bugzilla.redhat.com/show_bug.cgi?id=1447179 and the
ansible documentation.

Solution: bring the ability to merge roles files. So now by default,
ceph-docker-common and ceph-common will go into all.yml.sample

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1447179
Signed-off-by: Sébastien Han <seb@redhat.com>