Fixing the problem in loading plugins

This patch fixes the problem in loading the plugins in devstack
by defining the "plugins" variable in "run_plugins" function of
devstack/functions-common file.

Change-Id: I9fb0e24bf1fd282931a8489e0a8ec0c9ea078520
Closes-Bug: 1408571
diff --git a/functions-common b/functions-common
index 56106b3..7a6c3a1 100644
--- a/functions-common
+++ b/functions-common
@@ -1803,6 +1803,9 @@
 function run_plugins {
     local mode=$1
     local phase=$2
+
+    local plugins="${DEVSTACK_PLUGINS}"
+    local plugin
     for plugin in ${plugins//,/ }; do
         local dir=${GITDIR[$plugin]}
         if [[ -f $dir/devstack/plugin.sh ]]; then