Django 1.11 ends its extended support in April 2020 (which is before
Ussuri release), so horizon drops Django 1.11 support in Ussuri.
tox envs for non-primary Django versions are no longer needed in tox.ini
as testing environments for non-primary Django versions are setup in
the zuul jobs now.
horizon>=17.1.0 is required to use Django 2.2.
requirements.txt is updated accordingly.
For more info. please refer [1], [2].
[1] https://review.opendev.org/#/c/700733/
[2] https://etherpad.openstack.org/p/Enable_Django22_support_in_Horizon_Plugin
Change-Id: I6f393b85306e514f9e00567dfd2b9940d6f26253