Fix get_plugin_packages when multiple plugins are in use

Due to missing space no plugin packages are installed in this case.

Change-Id: I56b105510e4d31a411f2531fdb20c4ddf1ebff20
Closes-Bug: #1530675
diff --git a/functions-common b/functions-common
index c3eef46..93c9489 100644
--- a/functions-common
+++ b/functions-common
@@ -1176,7 +1176,7 @@
     local package_dir=""
     for plugin in ${DEVSTACK_PLUGINS//,/ }; do
         local package_dir="$(_get_package_dir ${GITDIR[$plugin]}/devstack/files)"
-        files_to_parse+="$package_dir/$plugin"
+        files_to_parse+=" $package_dir/$plugin"
     done
     echo "$(_parse_package_files $files_to_parse)"
     $xtrace