horizon 15.3.1 release

meta:version: 15.3.1
meta:diff-start: -
meta:series: stein
meta:release-type: release
meta:pypi: no
meta:first: no
meta:release:Author: Sean McGinnis <sean.mcginnis@gmail.com>
meta:release:Commit: Sean McGinnis <sean.mcginnis@gmail.com>
meta:release:Change-Id: If013d4d1ecec13d1b6d5c09e5f1888d4c60a91a8
meta:release:Code-Review+1: Vishal Manchanda <manchandavishal143@gmail.com>
meta:release:Code-Review+2: Hervé Beraud <hberaud@redhat.com>
meta:release:Code-Review+2: Thierry Carrez <thierry@openstack.org>
meta:release:Workflow+1: Thierry Carrez <thierry@openstack.org>