Add python tox verify job

This is a generic verify job that can be used to test anything that uses
Tox to run tests. This is mostly a port of OpenDaylight's version of the
same job. Will be useful initially to test lftools and global-jjb but we
plan to migrate all of the OpenDaylight Python / Tox jobs over to this
template as well.

Change-Id: I10417db8fc78a599b794087d3f9f729658214359
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>