horizon 15.2.0 release

meta:version: 15.2.0
meta:diff-start: -
meta:series: stein
meta:release-type: release
meta:pypi: no
meta:first: no
meta:release:Author: Akihiro Motoki <amotoki@gmail.com>
meta:release:Commit: Akihiro Motoki <amotoki@gmail.com>
meta:release:Change-Id: I42fcd8fc2702ab8e31e05aac0a3b00e62a7933e7
meta:release:Code-Review+1: Hervé Beraud <hberaud@redhat.com>
meta:release:Code-Review+2: Sean McGinnis <sean.mcginnis@gmail.com>
meta:release:Code-Review+2: Thierry Carrez <thierry@openstack.org>
meta:release:Workflow+1: Thierry Carrez <thierry@openstack.org>