Disable selinux for Fedora based distros

selinux locks down httpd which causes various issues for Horizon
working from git checkouts.  Dealing with selinux is more a deployment
than a development issue, so we just disable it for devstack.

Change-Id: I1e61f34e812360dafacd3b3288e66bc9cc5f1648
Fixes: 1175444