Use selinux=enforcing for Overcloud image builds
Use selinux=enforcing for Overcloud image builds

This will also be fixed in the unified CLI:
https://review.gerrithub.io/237539

I'm fixing it here as well just in case we have to fall back to this script for
any reason, and I don't want there to be a regression.

Change-Id: I9f1db761cf6ef5e02e79e6aed874a06d879b1e72
Related: rhbz#1233063

dmsimard/instack-undercloud,eprasad/instack-undercloud,bcrochet/instack-undercloud,dmsimard/instack-undercloud,rdo-management/instack-undercloud,rdo-management/instack-undercloud,dpkshetty/instack-undercloud,bcrochet/instack-undercloud,eprasad/instack-undercloud,dpkshetty/instack-undercloud