Add --label-opts options to allow user to customize container labels.

label-opts will allow you to customise 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)