Restrict release-wheel-mirror job to master
Restrict release-wheel-mirror job to master

Build and publish jobs for the wheel mirror only run against
master. Restrict the release job to master as well to avoid zuul
config errors due to stable branches not being able to run their
job depenendencies.

Without this we get many config error'd periodic stable jobs because
release-wheel-mirror cannot run its build and publish dependencies.

Change-Id: I6f26fe0e3451c07e6ea4f5267e28c014c6ba1ca8

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