Use constraints for api-ref environment
Use constraints for api-ref environment

Change [1] updated tox.ini to use constraints rather than
specific, non-constrained install commands but neglected
to update the api-ref environment.  In addition to fixing
that omission we also remove a bit of ugly trailing
whitespace.

Co-authored-By: Gorka Eguileor <geguileo@redhat.com>

[1] I879f34129c0d958be0a73bfdc7f641e178d4efe9

Change-Id: I58ec756cd3de2e501c77a4d947e680d0959d22ec

ge0rgi/cinder,Hybrid-Cloud/cinder,cloudbase/cinder,openstack/cinder,Nexenta/cinder,Hybrid-Cloud/cinder,phenoxim/cinder,openstack/cinder,Nexenta/cinder,NetApp/cinder,mahak/cinder,mahak/cinder,eharney/cinder,Datera/cinder,NetApp/cinder,phenoxim/cinder,cloudbase/cinder,eharney/cinder,Datera/cinder,j-griffith/cinder,j-griffith/cinder