RmExposedPorts to remove parent's exposed ports

runconfig.Merge can remove exposed ports from the final configuration if the user
specifies `Config.RmExposedPorts`

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
builder supports RMEXPOSE instruction

RMEXPOSE unexposed a list of ports inherited from the parent images.
For example if the parent image has:
> EXPOSE 8080 3030
and the child image has:
> UNEXPOSE 3030
The final image output will expose only port 8080

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)