[3.2] [PRD-2485] Add support for vlan splinters

Add option use_vlan_splinters to site.pp. If enabled and kernelmajversion
 < 3.3 then vlan splinters will be enabled if the driver for an bridge
 interface is present in the devices list in l23network::l2::splinters.

This behavior is disabled by default as not all installs may suffer from this
 and should only be enabled when necessary.