mdss: do not make pg_num a mandatory params

mdss: do not make pg_num a mandatory params

When playing ceph-mds role, mon nodes have set a fact with the default
pg num for osd pools, we can simply default to this value for cephfs
pools (`cephfs_pools` variable).

At the moment the variable definition for `cephfs_pools` looks like:

```
cephfs_pools:
  - { name: "{{ cephfs_data }}", pgs: "" }
  - { name: "{{ cephfs_metadata }}", pgs: "" }
```

and we have a task in `ceph-validate` to ensure `pgs` has been set to a
valid value.

We could simply avoid this check by setting the default value of `pgs`
to `hostvars[groups[mon_group_name][0]]['osd_pool_default_pg_num']` and
let to users the possibility to override this value.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1581164
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>