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/settings b/devstack/settings
new file mode 100644
index 0000000..3003ecf
--- /dev/null
+++ b/devstack/settings
@@ -0,0 +1,3 @@
+GITREPO["telemetry-tempest-plugin"]=${TELEMETRY_TEMPEST_REPO:-${GIT_BASE}/openstack/telemetry-tempest-plugin.git}
+GITDIR["telemetry-tempest-plugin"]=$DEST/telemetry-tempest-plugin
+GITBRANCH["telemetry-tempest-plugin"]=master