Add devstack plugin support

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

Change-Id: I3015dbc204ea3e4cd2592c03bbda3d398f4d050a

devstack/README.rst

@@ -0,0 +1,17 @@
+====================
+Enabling in Devstack
+====================
+
+1. Download DevStack::
+
+    git clone https://opendev.org/openstack/devstack.git
+    cd devstack
+
+2. Add this repo as an external repository::
+
+     > cat local.conf
+     [[local|localrc]]
+     enable_plugin octavia-tempest-plugin https://opendev.org/openstack/octavia-tempest-plugin
+
+3. run ``stack.sh``
+

devstack/plugin.sh

@@ -0,0 +1,14 @@
+
+# install_octavia_tempest_plugin
+function install_octavia_tempest_plugin {
+    setup_dev_lib "octavia-tempest-plugin"
+}
+
+if [[ "$1" == "stack" ]]; then
+    case "$2" in
+        install)
+            echo_summary "Installing octavia-tempest-plugin"
+            install_octavia_tempest_plugin
+            ;;
+    esac
+fi

devstack/settings

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+GITREPO["octavia-tempest-plugin"]=${OCTAVIA_TEMPEST_REPO:-${GIT_BASE}/openstack/octavia-tempest-plugin.git}
+GITDIR["octavia-tempest-plugin"]=$DEST/octavia-tempest-plugin
+GITBRANCH["octavia-tempest-plugin"]=${OCTAVIA_TEMPEST_REF:-master}