Fix provider networking error message

This fix replaces Q_USE_PROVIDERNET_FOR_PUBLIC with
Q_USE_PROVIDER_NETWORKING in the error messages introduced by
[1].

The error is thrown when provider networking with IPv6 has been
requested via local.conf, but no provider IPv6 range or provider
IPv6 gateway is provided. But if a provider network should be used
over the private network is determined along the variable
Q_USE_PROVIDER_NETWORKING and not Q_USE_PROVIDERNET_FOR_PUBLIC.

The variable Q_USE_PROVIDERNET_FOR_PUBLIC determines if a provider
network should be used as public network. This happens a few lines
later in the code and is not related to those error messages.

[1] https://review.openstack.org/#/c/326638/1/lib/neutron_plugins/
services/l3

Change-Id: I50aa1e9d2027eef598c95404851e51c31a397fbb
diff --git a/lib/neutron_plugins/services/l3 b/lib/neutron_plugins/services/l3
index 408b322..14928fb 100644
--- a/lib/neutron_plugins/services/l3
+++ b/lib/neutron_plugins/services/l3
@@ -191,8 +191,8 @@
         fi
 
         if [[ "$IP_VERSION" =~ .*6 ]]; then
-            die_if_not_set $LINENO IPV6_PROVIDER_FIXED_RANGE "IPV6_PROVIDER_FIXED_RANGE has not been set, but Q_USE_PROVIDERNET_FOR_PUBLIC is true and IP_VERSION includes 6"
-            die_if_not_set $LINENO IPV6_PROVIDER_NETWORK_GATEWAY "IPV6_PROVIDER_NETWORK_GATEWAY has not been set, but Q_USE_PROVIDERNET_FOR_PUBLIC is true and IP_VERSION includes 6"
+            die_if_not_set $LINENO IPV6_PROVIDER_FIXED_RANGE "IPV6_PROVIDER_FIXED_RANGE has not been set, but Q_USE_PROVIDER_NETWORKING is true and IP_VERSION includes 6"
+            die_if_not_set $LINENO IPV6_PROVIDER_NETWORK_GATEWAY "IPV6_PROVIDER_NETWORK_GATEWAY has not been set, but Q_USE_PROVIDER_NETWORKING is true and IP_VERSION includes 6"
             if [ -z $SUBNETPOOL_V6_ID ]; then
                 fixed_range_v6=$IPV6_PROVIDER_FIXED_RANGE
             fi