Remove discover from test-requirements
Remove discover from test-requirements

It's only needed for python < 2.7 which is not supported

Change-Id: I08d764ca74df4a91d37171a7c9a2419d787f7434

stackforge/tacker,openstack/tacker,zeinsteinz/tacker,openstack/tacker,stackforge/tacker,openstack/tacker,zeinsteinz/tacker