Add option to add config under [cinder-ceph]

Add option to add config under [cinder-ceph], currently, you can only add custom values under [DEFAULTS] however when I tried to enable image_volume_cache it needs to be added under a backed where you want to use it, [cinder-ceph] in my example. In order not to change cinder.conf manually so Juju won't wipe my change if I do any juju config, I've tried to do cinder.conf.d similar to what you can do with nova.conf.d. However, I don't think cinder search for conf.d as I couldn't get it to work only when I manually added the following changes in cinder.conf I've got cached enabled.

cinder_internal_tenant_project_id=910338c2886f465bb2d567148b0dac13
cinder_internal_tenant_user_id=edb967c574dd4c66becb55989a623c25
image_volume_cache_max_size_gb=300
image_volume_cache_max_count=10
[cinder-ceph]
image_volume_cache_enabled = True

It would be good if you could add some sort regex or search that if the following section/key-value pair is found then replace it or add it, in this case, you could have your extra customization within Juju deployment.