change Quantum network plugin set part

Currently in devstack, Q_PLUGIN can be set to only one of well-known quantum plugins ("openvswitch", "linuxbridge", "ryu").
Setting it to a custom plugin causes stack.sh to exit with an error.
This commit lets you run devstack with Q_PLUGIN set to a custom Quantum plugin.

Change-Id: Iafe07cf162b5b07bee120b0c571149c374004bb3
Signed-off-by: Takaaki Suzuki <suzuki@midokura.com>
diff --git a/stack.sh b/stack.sh
index 9db1262..f0f9410 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1266,8 +1266,10 @@
         Q_PLUGIN_CONF_FILENAME=ryu.ini
         Q_DB_NAME="ovs_quantum"
         Q_PLUGIN_CLASS="quantum.plugins.ryu.ryu_quantum_plugin.RyuQuantumPluginV2"
-    else
-        echo "Unknown Quantum plugin '$Q_PLUGIN'.. exiting"
+    fi
+
+    if [[ $Q_PLUGIN_CONF_PATH == '' || $Q_PLUGIN_CONF_FILENAME == '' || $Q_PLUGIN_CLASS == '' ]]; then
+        echo "Quantum plugin not set.. exiting"
         exit 1
     fi