detox's own requirements.txt limits the version of tox that it pulls
in. This can often conflict with the explicit version we set.
Let's use the version of tox as required by detox.
Issue: https://jira.opendaylight.org/browse/RELENG-136
Change-Id: I0b98afc6c993279c17cdd02d67471fadde23e552 Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>