Add ability to configure tempest tests based on network extensions

This patch introduces a Devstack variable to specify which network
extensions are enabled; this is useful for configuring tempest runs
when third-party plugins that do not support certain extensions.

Closes-bug: #1247778
Closes-bug: #1231152

Change-Id: Iee170993cb164502774f9ac4201b963d9a2715ba
diff --git a/lib/tempest b/lib/tempest
index ec1fc90..0af9310 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -299,6 +299,9 @@
     # cli
     iniset $TEMPEST_CONF cli cli_dir $NOVA_BIN_DIR
 
+    # Networking
+    iniset $TEMPEST_CONF network-feature-enabled api_extensions "${NETWORK_API_EXTENSIONS:-all}"
+
     # service_available
     for service in nova cinder glance neutron swift heat horizon ceilometer; do
         if is_service_enabled $service ; then