papi: Revert vpp-api-python to py2, add py3 pkg

papi: Revert vpp-api-python to py2, add py3 pkg

A previous commit, ca3d38ae18c21ce37212f0d5d5f4a1702429e70c, converts
the vpp-api-python package from a python2 library to a python3 one.
This changes the behaviour of the package and breaks anything that
uses it as a dependency.

This change reverts the package behaviour to a python2 package, and
additionally adds a *separate* python3 package for correct python3
behaviour.

Type: fix
Fixes: VPP-1738
Signed-Off-By: Ian Wells <iawells@cisco.com>
Change-Id: I54eb45156eb6e00fc4cd5b48e7a0aa17fd2f43f7
Signed-off-by: Dave Barach <dave@barachs.net>