webui: Do not allow picking gre mode for cisco plugin

The plugin really only makes sense for vlan.

https://bugzilla.novell.com/show_bug.cgi?id=884548
Validate that cisco plugin is using vlan mode
Validate that networking mode is vlan or gre
Merge pull request #138 from vuntz/cisco-vlan

Only allow vlan for cisco plugin + add some validation