Remove trailing whitespace in setup.cfg

Change-Id: I8e1c191672413e7571958223cb2775734d8af242
Reviewed-on: https://review.openstack.org/30946
Reviewed-by: Mark McLoughlin <markmc@redhat.com>
Reviewed-by: Sean Dague <sean@dague.net>
Approved: Mark McLoughlin <markmc@redhat.com>
Tested-by: Jenkins