mon: pool creation and pgs

mon: pool creation and pgs

Since we introduced config_overrides we removed a lot of options from
the default template. In some cases, like mds pool, openstack pools etc
we need to know the amount of PGs required. The idea here is to skip the
task if ceph_conf_overrides.global.osd_pool_default_pg_num is not define
in your `group_vars/all.yml`.

Closes: #1145
Signed-off-by: Sébastien Han <seb@redhat.com>
Co-Authored-By: Guillaume Abrioux <gabrioux@redhat.com>