--selinux-enabled flag should be ignored on Disabled SELinux systems

On Fedora and RHEL we ship selinux-enabled flag in the docker.service config,
but if people setup the /var/lib/docker as btrfs and disable SELinux,
we should not block the daemon from running.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #8181 from rhatdan/selinux_btrfs

--selinux-enabled flag should be ignored on Disabled SELinux systems