add RmVolumes to config to remove parent's volumes

RmVolumes contains a map of volumes path that will be removed from the final
configurations

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

RMVOLUME can be used to remove volume mountpoints inherited from the parent's
image.
For example, if the parent has
> VOLUME /test1
> VOLUME /test2
And the child has
> RMVOLUME /test2
> VOLUME /test3
The final image will have volumes /test1 and /test3

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
fixme: add a note about rebase/update when https://github.com/docker/docker/pull/7994 is merged

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