Add Horizon element

Depends on: https://review.openstack.org/53349

Change-Id: Ic0dc4986b2f8f1d0bb66c74d17918581a591e9cd
Signed-off-by: Tomas Sedovic <tomas@sedovic.cz>
(cherry picked from commit b9270df148178050943556a53ff0e391903e6c10)
Install mariadb-devel for horizon

I couldn't build the horizon element without installing this.
Pin horizon to specific commit