core,restapi: attach detach storage connections

Introduce capability for attaching/detaching connections
for storage domains.

Core - new commands:
* AttachStorageConnectionToStorageDomainCommand
  (support existing/floating connections)
* DetachStorageConnectionFromStorageDomainCommand

REST:
* Attach:
  POST /api/storagedomains/{storage_id}/storageconnections
    <storage_connection id="{connection_id}">
    </storage_connection>
* Detach:
  DELETE /api/storagedomains/{storage_id}/storageconnections/{connection_id}

Feature page: http://www.ovirt.org/Features/Manage_Storage_Connections

Change-Id: I319bc9f51c81e2df0c7ed3b3338fcd7b4783fe84
Signed-off-by: Daniel Erez <derez@redhat.com>
Signed-off-by: Alissa Bonas <abonas@redhat.com>