Update delete_network_by_name in VApp OM to return self instead of nil

Change-Id: I0da8a69e85f14525026c25a83f15eaee7de4f320
Update delete_catalog_by_name method to return Client object instead of nil

Change-Id: I5404263470d4720bc99ea37a1f34895f03f2a5dc
Update delete_disk_by_name in VDC OM to return self instead of nil

Change-Id: I9db9d0359fc15683c6a7679258a2b59a426209c8