Update to tempest 20.0.0

Update to tempest 20.0.0

test_dualnet_multi_prefix_slaac which is uncovered (marked as slow) by
OpenStack gates [1] is failing on lf-virtual6 [2]. Updating to 20.0.0
is closer to the current OpenStack Neutron Rocky gates
(tempest==20.0.1.dev42) [3].

It's worth mentioning that we cannot downgrade to 19.0.0 because the
next commit is needed [4].

[1] http://logs.openstack.org/28/656428/1/check/tempest-multinode-full-py3/74d9cd7/testr_results.html.gz
[2] http://artifacts.opnfv.org/functest/functest-opnfv-functest-smoke-hunter-tempest_scenario-run-113/results/tempest_scenario/tempest-report.html
[3] http://logs.openstack.org/57/654957/2/check/neutron-tempest-multinode-full/e5eda83/job-output.txt.gz
[4] https://github.com/openstack/tempest/commit/3588bb3f5ef546a0ef4d4ad621fd1be381b5fdaf

Change-Id: I496df1272f26ad73daf388070b762b736e4e5fd8
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>