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
Additional sanity checks for RHEL6

Simple check to see if EPEL repo is enabled for RHEL6.  Installing
repos such as EPEL may be a personal preference, so we give the user
the URL to fix it rather than any automated action.

Also adds an additional sanity check which checking for pip.  In the
prior case of EPEL being disabled, a RHEL6 host may not have access to
python-pip.  Although this shouldn't happen, its a good sanity check.

Change-Id: I4e8a4dda9475b75d071534d4eef469198502a048