[RFE] Allow using a volume for Manila instance root disk

[RJ]

Description
===========

As it stands, there doesn't seem to be any way to use a Cinder volume for the root disk of a Manila VM instance. This is inconsistent with other OpenStack projects, such as Octavia and Magnum, where it is possible to configure a volume. I've chosen to architect my system without any local storage for Nova, so this is currently preventing me from deploying shares without overhauling my storage configuration.

Steps to reproduce
==================

* Deploy OpenStack / Manila, with local storage disabled for Nova.

  ```title="/etc/kolla/nova-compute/nova.conf"
  [DEFAULT]
  max_local_block_devices = 0
  ```

* Create a Manila flavor with "Root Disk" size of 0 (no local storage).
* Attempt to configure `manila-share/manila.conf` to use a volume for the root disk.

Expected result
===============

Be able to configure `manila-share/manila.conf`, similar to `octavia-worker/octavia.conf`:

```title="/etc/kolla/octavia-worker/octavia.conf"
[controller_worker]
volume_driver = volume_cinder_driver

[cinder]
region_name = RegionOne
endpoint_type = internal
ca_certificates_file =
volume_type = PREMIUM
volume_size = 5
```

Actual result
=============

No such options seem to exist in the `manila.conf` reference. Receive error message when trying to create Manila NFS share (see log message below).

Environment
===========

OpenStack / Manila 2023.2 (Bobcat) using Generic backend (LVM-backed). Neutron with OpenVSwitch.

Logs & Configs
==============

https://paste.opendev.org/show/824571/

[Vida Haririan (vhariria)]

See additional triage information at https://meetings.opendev.org/meetings/manila/2024/manila.2024-07-04-15.00.log.txt