Added devstack plugin for vitrage-tempest-plugin

* In order to make the ease of installation, devstack plugin is
  added to install the tempest plugin.

Change-Id: I386b834627493df8dc2c3dcd597fa5aa1cc9467f

devstack/README.rst

@@ -0,0 +1,21 @@
+====================
+Enabling in Devstack
+====================
+
+**WARNING**: the stack.sh script must be run in a disposable VM that is not
+being created automatically, see the README.md file in the "devstack"
+repository.  See contrib/vagrant to create a vagrant VM.
+
+1. Download DevStack::
+
+    git clone https://git.openstack.org/openstack-dev/devstack.git
+    cd devstack
+
+2. Add this repo as an external repository::
+
+     > cat local.conf
+     [[local|localrc]]
+     enable_plugin vitrage-tempest-plugin https://git.openstack.org/openstack/vitrage-tempest-plugin
+
+3. run ``stack.sh``
+

devstack/plugin.sh

@@ -0,0 +1,13 @@
+# install_vitrage_tempest_plugin
+function install_vitrage_tempest_plugin {
+    setup_dev_lib "vitrage-tempest-plugin"
+}
+
+if [[ "$1" == "stack" ]]; then
+    case "$2" in
+        install)
+            echo_summary "Installing vitrage-tempest-plugin"
+            install_vitrage_tempest_plugin
+            ;;
+    esac
+fi

devstack/settings

@@ -0,0 +1,3 @@
+GITREPO["vitrage-tempest-plugin"]=${VITRAGE_TEMPEST_REPO:-${GIT_BASE}/openstack/vitrage-tempest-plugin.git}
+GITDIR["vitrage-tempest-plugin"]=$DEST/vitrage-tempest-plugin
+GITBRANCH["vitrage-tempest-plugin"]=master