Add volume_retype module

Introduce openstack.cloud.volume_retype, a new module that wraps
the Cinder os-retype block storage action (POST /volumes/{id}/action)
to change the type of an existing volume.

The module supports both migration policies exposed by the Cinder API:
- 'never'     – changes volume type metadata only; requires source and
                target types to share the same storage backend.
- 'on-demand' – triggers a full data migration to the backend
                associated with the new volume type.

The module is idempotent: if the volume already carries the requested
type no API call is made and changed=false is returned. It also
honours Ansible check mode, resolving the volume and target type to
verify feasibility without mutating any state.

A wait/timeout mechanism is included. For on-demand retypes the module
polls migration_status until 'success' or 'error'. For never retypes
it polls volume status until 'available'.

The equivalent CLI command is:
  openstack volume set --type <type> --retype-policy <policy> <vol>

This closes a gap in the collection: openstack.cloud.volume handles
create/delete/update but does not expose the retype action.

Integration tests are provided covering: basic retype, idempotency,
check mode, lookup by UUID, and error handling for missing volume
and missing volume type.

Change-Id: I32b2b6303366a19baa1e3f1473b09650f6daee66
Signed-off-by: Simon Dodsley <simon@purestorage.com>

meta/runtime.yml

@@ -101,3 +101,4 @@ action_groups:
     - volume_snapshot_info
     - volume_type_access
     - volume_image_metadata
+    - volume_retype