osd: add pg autoscaler support

osd: add pg autoscaler support

This commit adds the pg autoscaler support.

The structure for pool definition has now two additional attributes
`pg_autoscale_mode` and `target_size_ratio`, eg:

```
test:
  name: "test"
  pg_num: "{{ osd_pool_default_pg_num }}"
  pgp_num: "{{ osd_pool_default_pg_num }}"
  rule_name: "replicated_rule"
  application: "rbd"
  type: 1
  erasure_profile: ""
  expected_num_objects: ""
  size: "{{ osd_pool_default_size }}"
  min_size: "{{ osd_pool_default_min_size }}"
  pg_autoscale_mode: False
  target_size_ratio": 0.1
```

when `pg_autoscale_mode` is `True` user has to set a decent value in
`target_size_ratio`.

Given that it's a new feature, it's still disabled by default.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1782253
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>