Set cinder API list limit to 6
Set cinder API list limit to 6

We want to be able to test pagination, but the current limit before
pagination starts is 1000. We don't want to try to make 1001 volumes in
devstack. Set it to 6.

Change-Id: Iaeeae708a02fd630f6ad1a5f1cff3cc1045347ec

openstack-infra/project-config,openstack-infra/project-config