Fix README
Fix README

The oslo config-generator-tempest.conf location is defined
as still being:

'etc/config-generator.tempest.conf'

But after the changes made in tempest-run-cmd bp the location
changed to:

'tempest/cmd/config-generator.tempest.conf'.

Change-Id: I53b0ab18a217eb186c51eb5a2e35489967fc5c55
Closes-Bug: #1606586

vedujoshi/tempest,openstack/tempest,cisco-openstack/tempest,sebrandon1/tempest,Juniper/tempest,Tesora/tesora-tempest,openstack/tempest,vedujoshi/tempest,masayukig/tempest,cisco-openstack/tempest,Tesora/tesora-tempest,sebrandon1/tempest,masayukig/tempest,Juniper/tempest