common: support setting pg autoscaler to off

common: support setting pg autoscaler to off

The current implementation doesn't allow to disable the pg autoscaler
on created pools. This allows only 'on' or 'warn'.

With this commit, this is now possible to disable it.

Valid values would be ['on', 'yes', 'true', 'off', 'no', 'false']

```
openstack_glance_pool:
  name: "images"
  pg_num: 128
  pgp_num: 128
  rule_name: "replicated_rule"
  type: 1
  application: "rbd"
  size: 3
  pg_autoscale_mode: off
```

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2062621
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>