Update tox.ini for new upper constraints strategy

The requirements team has defined a new strategy for handling
upper constraints[1]. This patch applies those strategies to
Octavia.

[1] http://lists.openstack.org/pipermail/openstack-discuss/ \
    2019-May/006478.html

Change-Id: I38a5bedbdd7852bd6e7bfff4917634458a6edbf3
1 file changed