restapi: use storageconnection instead of storage

Change the code to use StorageConnection object instead of Storage
in storage connections REST resource.
Fix tests,rsdl,api.xsd accordingly.

Change-Id: Icb401cef8e08e62c57c0ef8afd4677c764d065a9
Signed-off-by: Alissa Bonas <abonas@redhat.com>