Add label.InitLabels functioni. Allows generation of labels based on options

This will allow us to do the following with docker.

Customize the way that a labeling system like SELinux will run on a container.

    --label-opt="user:USER"  : Set the label user for the container
    --label-opt="role:ROLE"  : Set the label role for the container
    --label-opt="type:TYPE"  : Set the label type for the container
    --label-opt="level:LEVEL"  : Set the label level for the container
    --label-opt="disabled"  : Turn off label confinement for the container

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
How does disable work? We clear processLabel and mountLabel, but reset them back to pcon and mcon after exiting the loop. How does clearing these labels help? Can you add some comments on the expected result. A test would be even better :)