Update disk_exists? method to call list_disks instead of disks

Change-Id: Ie0d40612c7a73bdb7128a1fb57a42bcde84909fb
Add NIC OM and nics method in VM OM

Change-Id: Ibe0a7d9b8bbf7b0a9c7960c6cc55a18382140e37
Remove list_networks method in VM OM

Change-Id: I03251a1744eb9d6274be45f9b2a0b5fdf46d88a9