Change `docker service update` to replace attributes of the target
service rather than augment them. One particular occurrence where the
previous behavior proved problematic is when trying to update a port
mapping: the merge semantics provided no way of removing published
ports, but strictly of adding more.
The utility merge* functions where renamed accordingly to update*.
Signed-off-by: Arnaud Porterie (icecrime) <arnaud.porterie@docker.com>
