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)
nit: Would it be possible to expand all 'Con' suffixes to Context? We seem to be using both.
As a selinux noob, I am still not sure when to use Con vs Label :)