Add a 64-byte interface "tag" for vhost and tap interfaces

Add a 64-byte interface "tag" for vhost and tap interfaces

This patch should dispose of spurious objections around interface tag
requirements, currently in use as excuses not to support the vpp ML2
plugin.

Add "u8 tag[64];" to the sw_interface_details message sent by vpp to
control-plane clients. Add u8 tag[64] to the create_vhost_user_if and
tap_connect APIs.

Added debug CLI to set/show/clear the interface tag on any vnet sw
interface. Added the sw_interface_tag_add_del API to set/clear
tags on any vnet sw interface.

There can be no expectation of "tag atomicity" with respect to
physical hardware. Vpp discovers devices before establishing a
control-plane connection.

This patch upload verifies using the csit oper-161128 branch

Change-Id: If8520119e7a586c5ccf0fdda82484ac205622855
Signed-off-by: Dave Barach <dave@barachs.net>