restapi: add connections subresource for domain

Adding storage connections subresource for domains.
This subresource returns storage connections that
are in use by a specific domain.
Available on url:
api/storagedomains/<domainId>/storageconnections
and:
api/storagedomains/<domainId>/storageconnections/<connectionId>
Exposing GET only, no add/delete/update.
Feature page:
http://www.ovirt.org/Features/Manage_Storage_Connections

Change-Id: I0f63f8384c245eb28c18a3fde87ba57a8c1cd678
Signed-off-by: Alissa Bonas <abonas@redhat.com>