Swarm mode makes it possible through the API to set labels to containers
but not through command line. This tries to fix it.
Signed-off-by: Vincent Demeester <vincent@sbr.pm>
(cherry picked from commit 4031d70d1be2f02e1c9d6f7738f2a816be7eeef2)
Signed-off-by: Tibor Vass <tibor@docker.com>