Convert lisp-gpe encap to interface tx node

Convert lisp-gpe encap to interface tx node

With this change, one lisp-gpe interface is created per vrf/overlay
tenant and its tx node is used as encapsulator (or tunnel ingress). For
all intents and purposes, the tx node inherits all functions previously
performed by the lisp-gpe-encap node and it maintains said node's
position in lisp-gpe's data-path graph. Chiefly, this opens the
possibility to chain interface features, like IPSec, transparently with
LISP. Furthermore, it brings basic data plane support for vrfs and LISP
instance-ids (or virtual network instances as per RFC7364).

Other changes include improvements to lisp-gpe enable and disable
sequences and corresponding API/VAT fixes.

Change-Id: I085500450660a976b587b1a720e282f6e728d580
Signed-off-by: Florin Coras <fcoras@cisco.com>