The type of VLAN encapsulation desired to be used on this
trunk port. It is either a particular type, or 'negotiate'
meaning whatever type results from the negotiation.
negotiate(5) is not allowed if the port does not support
negotiation or if its vlanTrunkPortDynamicState is set to
on(1) or onNoNegotiate(5). Whether writing to this object
in order to modify the encapsulation is supported is both
device and interface specific.