Add builtin nodes discovery

Use `pkg/discovery` to provide nodes discovery between daemon instances.

The functionality is driven by two different command-line flags: the
experimental `--cluster-store` (previously `--kv-store`) and
`--cluster-advertise`. It can be used in two ways by interested
components:

1. Externally by calling the `/info` API and examining the cluster store
   field. The `pkg/discovery` package can then be used to hit the same
   endpoint and watch for appearing or disappearing nodes. That is the
   method that will for example be used by Swarm.
2. Internally by using the `Daemon.discoveryWatcher` instance. That is
   the method that will for example be used by libnetwork.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

docs/reference/commandline/daemon.md

@@ -22,6 +22,8 @@ weight=1
       -D, --debug=false                      Enable debug mode
       --default-gateway=""                   Container default gateway IPv4 address
       --default-gateway-v6=""                Container default gateway IPv6 address
+      --cluster-store=""                     URL of the distributed storage backend
+      --cluster-advertise=""                 Address of the daemon instance to advertise
       --dns=[]                               DNS server to use
       --dns-opt=[]                           DNS options to use
       --dns-search=[]                        DNS search domains to use
@@ -484,6 +486,12 @@ Be careful setting `nproc` with the `ulimit` flag as `nproc` is designed by Linu
 set the maximum number of processes available to a user, not to a container. For details
 please check the [run](run.md) reference.
 
+## Nodes discovery
+
+`--cluster-advertise` specifies the 'host:port' combination that this particular
+daemon instance should use when advertising itself to the cluster. The daemon
+should be reachable by remote hosts on this 'host:port' combination.
+
 ## Miscellaneous options
 
 IP masquerading uses address translation to allow containers without a public