[CentOS] [CentOS7] switching workspace on external monitor

Hi,

I just installed CentOS 7 on my NB and I'm experiencing problem during 
switching the workspace in GNOME3 environment. The workspace on my NB 
display is working correctly after change from one workspace to another. 
But the windows placed on the external/second display stay on the active 
workspace.

I found the description and solution of this problem here:
http://gregcor.com/2011/05/07/fix-dual-monitors-in-gnome-3-aka-my-workspaces-are-broken/
but I can't find the key "workspaces_only_on_primary" in my installation.

Does anybody have a tip for solving this problem?

Thanks

martin