Work on 2014-06-01

Tags: openstack

Fix broken link in openstack-infra/config/doc/source/devstack-gate.rst

The “At a Glance” section links to devstack_launch_slave.pp, which was deleted.

• Bugs go here for openstack-infra/config: https://bugs.launchpad.net/openstack-ci
• Bug at: https://bugs.launchpad.net/openstack-ci/+bug/1325379
• Review at: https://review.openstack.org/#/c/97110/