Add pip::python3 module and pip3 provider.

Adds python3 support to our pip package. Also adds a custom version
of the pip package provider which can be used when installing
things w/ python3-pip is explicitly required. For example:

  package { 'tox',
    provider => pip3,
    require  => Class[pip::python3],
  }

Change-Id: I1b563c8f7e647dad3a134ef1f0ec5b45b63f8b71
Reviewed-on: https://review.openstack.org/32451
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins