Add python3 option to jenkins::slave.

Adds a new python3 option to our jenkins slave manifest which
can be used to enable using python3-pip when installing
*key* pip packages for unit testing (tox, setuptools-git).

Also, updates openstack_project::slave so that it supports the option
as well.

Change-Id: I5face30b09e3e8d2f70b4da83481f4f8bd33fc51
Reviewed-on: https://review.openstack.org/32452
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins