Ability to import an image in multiple stores

Despite using the option enabled_backends, we can only import an image one time and in one backend.
The 2 first problems pointed out by the multi-store backend support specification (https://specs.openstack.org/openstack/glance-specs/specs/rocky/approved/glance/multi-store.html) are not fully fulfilled with the current implementation:

- It’s a manual out-of-band task to copy image bits to secondary storage.
- The operator must manage store locations manually; there is no way to query the available stores to back an image’s bits in glance.

We should be able to import an image in multiple stores through the glance api and without manual tasks.
For example by allowing users to specify multiple backend at the image upload, we could leverage on the current interoperable import workflow to send data in all the specified stores.