Update neutron_available config option.

Tempest change I5ee9ec816845de483fe88d76d1bb047e7bb1af7e changed
the behavior of the neutron_available config option. This commit
updates devstack to use it's new name and group.

Change-Id: I7d5074209fe81f6100f380512d7702fbc8e252ac
diff --git a/lib/tempest b/lib/tempest
index 0e066a8..b4a579b 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -253,7 +253,7 @@
 
     # Network
     if is_service_enabled neutron; then
-        iniset $TEMPEST_CONF network neutron_available "True"
+        iniset $TEMPEST_CONF service_available neutron "True"
     fi
     iniset $TEMPEST_CONF network api_version 2.0
     iniset $TEMPEST_CONF network tenant_networks_reachable "$tenant_networks_reachable"