Add NIC config for compute role for DVR with multiple NICs

Add NIC config for compute role for DVR with multiple NICs

This change adds a NIC config to the multiple-nics sample NIC
config templates for a compute node running DVR. In order for
DVR to work on the compute nodes, they must share an external
bridge with the controllers. All of the other sample NIC
configs already have an external bridge (defaults to 'br-ex'),
but the multiple-nics compute role does not, so now the
compute-dvr.yaml NIC template will demonstrate DVR with
multiple NICs.

Change-Id: I80fe2e5842a67984e1d4d8aa295c7607c4f340ad