Cap Pip<8 due to pip bug
Cap Pip<8 due to pip bug

Due to https://github.com/pypa/pip/issues/3384 in pip v8, this change
puts a cap on pip, in a similar manner to
https://review.openstack.org/#/c/269954/

Change-Id: I20b8b597e8806a7ee21d947eccc98e4ae7e00c3b
Closes-Bug: 1536290

openstack/tempest,sebrandon1/tempest,masayukig/tempest,LIS/lis-tempest,bigswitch/tempest,Juniper/tempest,zsoltdudas/lis-tempest,zsoltdudas/lis-tempest,Tesora/tesora-tempest,sebrandon1/tempest,Juniper/tempest,masayukig/tempest,vedujoshi/tempest,cisco-openstack/tempest,cisco-openstack/tempest,Tesora/tesora-tempest,vedujoshi/tempest,openstack/tempest,LIS/lis-tempest,bigswitch/tempest