Rally test of NovaServers.boot_server_attach_created_volume_and_live_migrate
requires 10Gb volume to function, so extend volume group size in order to
avoid insufficient free space error.
Change-Id: I317935d2bf736db360061e24f299c9b885154a9e
Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>
(cherry picked from commit 
2d95dee51177e66f5370b85115c2999925b13657)
     external_interface: ${_param:opnfv_fn_vm_quaternary_interface}
     interface_mtu: 9000
     linux_system_codename: xenial
-    loopback_device_size: 10
+    loopback_device_size: 20
   nova:
     compute:
       libvirt_service: libvirtd