Enable nbd if you're running an lxc virt_type without an lvm backend
Enable nbd if you're running an lxc virt_type without an lvm backend

If nbd isn't enabled you can't boot instances with libvirt using lxc
(unless you're using an lvm backend).

Closes-Bug: #1487195

Co-Authored-By: Andrew Melton <andrew.melton@rackspace.com>

Change-Id: I08c4d498ed35166f566291d9530ca1ecfae05625

bswartz/devstack,noironetworks/devstack,TwinkleChawla/devstack,thomasem/devstack,vsham20/devstack,noironetworks/devstack,openstack-dev/devstack,dtroyer/devstack,costingalan/devstack,atulpatil301/devstack,olivierlemasle/devstack,costingalan/devstack,dirkmueller/devstack,parul24/devstack,parul24/devstack,pombredanne/devstack,atulpatil301/devstack,richliu/devstack,TwinkleChawla/devstack,prithivm/branch01,group-policy/devstack,ozamiatin/devstack,varunarya10/devstack,prithivm/branch01,vsham20/devstack,mssumanth/devstack,Mahito/devstack,Mahito/devstack,bigswitch/devstack,bswartz/devstack,thomasem/devstack,group-policy/devstack,nawawi/openstack,pczerkas/devstack,mc2014/devstack,richliu/devstack,openstack-dev/devstack,avvocatodemarchis/devstack,olivierlemasle/devstack,pombredanne/devstack,samgoon/devstack,avvocatodemarchis/devstack,samgoon/devstack,pczerkas/devstack,dirkmueller/devstack,ozamiatin/devstack,dtroyer/devstack,varunarya10/devstack,mc2014/devstack,mssumanth/devstack,nawawi/openstack,bigswitch/devstack