Handle base images that have selinux disabled

The CentOS 6 base image has selinux set to disabled. Because of this, it
requires a stepping through permissive with a relabel during a reboot.
Unfortunately you can't have the scripts issue the reboot without
vagrant getting annoyed.

Change-Id: Ia46348162df9dc5deabf3a6d1907ca54cbd97b33
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>