Peer discovery with swarmkit API for Docker plugin v2

[lukemarsden]

Currently if you docker plugin install weave net comes up on every node but the nodes don't know how to find eachother. Implement this by copying the way weave-kube does it but plugging it into the swarm nodes API instead.

cc @bboreham

[brb]

The list of approaches I considered:

1. To use GET /nodes (Docker API reference) to get a list of nodes to connect to when starting the plugin. Unfortunately, /nodes can be called only from a master node, so it cannot be used when starting the plugin on a worker node.
2. Accidentally I found out that GET /info returns the list of master nodes. So, when starting the plugin on a worker, we pass -ipalloc-init observer, while on a master node we pass --ipalloc-init consensus=$NUMBER_OF_MASTERS. In both cases, we pass the list of master peers to weave to connect to. It works fine, but it's not clear whether it is safe to use the info from /info. See Undocumented field in GET /info response moby/moby#31386.
3. To use GET /nodes and DiscoverNew events which the plugin receives when a node joins the Swarm cluster.

I implemented the 2nd one, because of the simplicity, and because at the time of testing I couldn't receive DiscoverNew events. However, after cleaning up my testing env I am able to receive the events. Depending on moby/moby#31386 I might need to implement the 3rd option.

[marccarre]

Fixed by #2727.