Disable selinux for RHEL6

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
Merge "Disable selinux for RHEL6"
Merge branch 'master' of github.com:openstack-dev/devstack