Install pymemcache in devstack when needed

In case VITRAGE_COORDINATION_URL is set to memcached, we need to
ensure that all dependencies are met inside the venv.

Change-Id: I78353ccec3074a10e10dad5c6583456016d962a1

devstack/plugin.sh

@@ -273,6 +273,10 @@ function install_vitrage {
     if [[ "$VITRAGE_DEPLOY" == "mod_wsgi" ]]; then
         install_apache_wsgi
     fi
+    # Install reqs for tooz driver
+    if [[ "$VITRAGE_COORDINATION_URL" =~ "memcached" ]]; then
+        pip_install_gr "pymemcache"
+    fi
 }
 
 # install_vitrageclient()