Make configure_neutron_nova_new and create_nova_conf_neutron param optional

The commit e95f2a36645b58b172855213cb8311a3486bfcd9 broke
networking-ovn (and potentially other ml2 drivers) by making the config
parameter mandatory. It doesn't need to be.

Change-Id: I0d5738ac3a6d27ddb7655835d77689409a6ff6f4
diff --git a/lib/neutron-legacy b/lib/neutron-legacy
index 2e8992d..bee032a 100644
--- a/lib/neutron-legacy
+++ b/lib/neutron-legacy
@@ -366,7 +366,7 @@
 }
 
 function create_nova_conf_neutron {
-    local conf="$1"
+    local conf=${1:-$NOVA_CONF}
     iniset $conf DEFAULT use_neutron True
     iniset $conf neutron auth_type "password"
     iniset $conf neutron auth_url "$KEYSTONE_AUTH_URI"