Add devstack plugin to ease integration tests

Instead of requiring users to install the plugin manually, add a
devstack plugin that can be enabled in order to perform the
installation.

Change the integration tests to use the globally installed plugin
instead of using TEMPEST_PLUGINS.

Change-Id: I3ac8516a0c15b0816c14074ec5119e317735edd6
diff --git a/devstack/plugin.sh b/devstack/plugin.sh
new file mode 100644
index 0000000..673c12a
--- /dev/null
+++ b/devstack/plugin.sh
@@ -0,0 +1,13 @@
+# install_telemetry_tempest_plugin
+function install_telemetry_tempest_plugin {
+    setup_dev_lib "telemetry-tempest-plugin"
+}
+
+if [[ "$1" == "stack" ]]; then
+    case "$2" in
+        install)
+            echo_summary "Installing telemetry-tempest-plugin"
+            install_telemetry_tempest_plugin
+            ;;
+    esac
+fi