This commit in conjunction with a libnetwork side commit,
cleans up the libnetwork SetClusterProvider logic interaction.
The previous code was inducing libnetwork to spawn several go
routines that were racing between each other during the agent
init and close.
A test got added to verify that back to back swarm init and leave
are properly processed and not raise crashes
Signed-off-by: Flavio Crisciani <flavio.crisciani@docker.com>
`docker network prune` prunes unused networks, including overlay ones.
`docker system prune` also prunes unused networks.
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
