Zuul gate window increment by 2, floor at 6

* modules/openstack_project/files/zuul/layout.yaml: Be more
optimistic overall with the gate window, increasing by 2 instead of
1 on successes and never falling below 6. This will allow us to
utilize more available test node quota, hopefully still without
falling into another starvation scenario.

Change-Id: I6d54c0a8bd0575d0c5a85ee93bd5af77534ff2bf