Remove hard coding of SELinux labels on systems without proper selinux policy.

If a system is configured for SELinux but does not know about docker or
containers, then we want the transitions of the policy to work.  Hard coding
the labels causes docker to break on older Fedora and RHEL systems

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
In certain cases, setting the process label will not happen.

When the code attempts to set the ProcessLabel, it checks if SELinux Is
enabled.  We have seen a case with some of our patches where the code
is fooled by the container to think that SELinux is not enabled.  Calling
label.Init before setting up the rest of the container, tells the library that
SELinux is enabled and everything works fine.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Fix SELinux issue with missing Contexts in lxc execdriver

There is a bug in the SELinux patch for the lxc execdriver, that
causes lxc containers to blow up whether or not SELinux is enabled.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge branch 'selinux' of https://github.com/rhatdan/docker into rhatdan-selinux

Conflicts:
	pkg/selinux/selinux.go
	runtime/execdriver/lxc/lxc_template.go

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Fix lxc label handleing

This also improves the logic around formatting the labels for selinux
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5002 from crosbymichael/rhatdan-selinux

Improve selinux label handling