Explicitly set use_neutron=False in nova.conf when running nova-net

nova-network has been deprecated since Netwon and Nova change
I8388c29ad310cd8800084b4d5c026013158bfbed is switching the default
value of use_neutron to True, so we need devstack to explicitly
set use_neutron=False when running and configuring nova-network.

Part of blueprint use-neutron-by-default

Change-Id: I82721b5d10711401b9b0ebc2b0ed07cc8287bbf7

lib/nova

@@ -663,6 +663,7 @@ function create_nova_conf_nova_network {
     if [ -n "$FLAT_INTERFACE" ]; then
         iniset $NOVA_CONF DEFAULT flat_interface "$FLAT_INTERFACE"
     fi
+    iniset $NOVA_CONF DEFAULT use_neutron False
 }
 
 # create_nova_keys_dir() - Part of the init_nova() process