Ensure SELinux is permissive on Ceph OSDs

Ensure SELinux is permissive on Ceph OSDs

Currently we build the overcloud image with selinux-permissive element
in CI. However, even in environments where selinux-permissive element is
not used, it should be ensured that SELinux is set to permissive mode on
nodes with Ceph OSD [1].

We have no nice way to manage SELinux status via Puppet at the moment,
so i'm resorting to execs, but with proper "onlyif" guards.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1241422

Change-Id: I31bd685ad4800261fd317eef759bcfd285f2ba80