image-volume ambiguous configuration leads to Quota exceeded for resources: ['volumes']

Hello,

Have two openstack clusters with cinder backed by pure storage arrays.
1st cloud:
Ubuntu Focal, OpenStack Ussuri, cinder charm revision 607 from ussuri/stable, cinder packages version 16.4.1-0ubuntu1, Juju "2.9.42".
2nd cloud:
Ubuntu Focal, OpenStack Ussuri, cinder charm revision 607 from ussuri/stable, cinder packages version 16.4.2-0ubuntu2.2, Juju "2.9.42".

The image-volume cache is enabled with "juju config cinder-volume image-volume-cache-enabled=true".
Every time OpenStack volume is created from an image, I see in cinder logs:
```
Failed to create new image-volume cache entry. Error: Quota exceeded for resources: ['volumes']
```
I create volumes with:
```
openstack volume create --image d906b8bd-1f7f-4e46-b01b-b147e97be33f --size 6 --type pure-c-array-iscsi foo-bar-volume
# Or a VM with bootable volume
openstack server create foo-bar-vm --flavor m1.medium --image ubuntu-20.04 --network network --boot-from-volume 6
```

I checked that cinder charm generates cinder.conf with:
```
cinder_internal_tenant_project_id = services
cinder_internal_tenant_user_id = cinder
```

However, in the documentation [1] it is recommended to use IDs.

For some reason, maybe because of some internal Keystone kitchen, we have two "services" projects on both clouds. And one of the projects has a "cinder" user and another one has none. And I checked a number of other Focal Ussuri and even newly deployed Jammy Yoga clouds, they all have two "services" projects generated by Juju.

I tried to update `cinder.conf` manually and put explicitly the ID of the "cinder" user and the ID of the project that has this user configured. And restarted the cinder-volume service. After that the "Quota exceeded for resources" vanished and the cache started working on both clouds, volumes are created in a split second.

As it looks to me the situation with two "services" projects might be quite uneasy to resolve. So the most preferable fix would be to specify the exact IDs for "internal_tenant".

[1] https://docs.openstack.org/cinder/latest/admin/image-volume-cache.html