FIX: incus_storage_bucket produces inconsistent result

[maveonair]

This pull request fixes the situation where incus_storage_bucket produces an inconsistent result when using the lvm storage driver, see https://github.com/lxc/terraform-provider-incus/issues/47.

The same solution is already used in other resources, e.g:

• https://github.com/lxc/terraform-provider-incus/blob/main/internal/storage/resource_storage_volume.go#L380-L385
• https://github.com/lxc/terraform-provider-incus/blob/main/internal/storage/resource_storage_pool.go#L337-L372

[stgraber]

We can probably share some of that logic with the volume issue we have open as both are basically the same where volume.XYZ keys set on the pool will propagate to new volumes or buckets.

[stgraber]

So basically it's safe to ignore any key the user didn't define so long as the key and value line up with the matching volume.XYZ key on the pool.