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>

hack/.vendor-helpers.sh

@@ -108,6 +108,8 @@ clean() {
 	findArgs=(
 		# This directory contains only .c and .h files which are necessary
 		-path vendor/src/github.com/mattn/go-sqlite3/code
+		# This directory is needed for compiling the unit tests
+		-o -path vendor/src/github.com/stretchr/objx
 	)
 	for import in "${imports[@]}"; do
 		[ "${#findArgs[@]}" -eq 0 ] || findArgs+=( -or )

hack/vendor.sh

@@ -16,6 +16,7 @@ clone git github.com/kr/pty 5cf931ef8f
 clone git github.com/mattn/go-sqlite3 v1.1.0
 clone git github.com/microsoft/hcsshim 7f646aa6b26bcf90caee91e93cde4a80d0d8a83e
 clone git github.com/mistifyio/go-zfs v2.1.1
+clone git github.com/stretchr/objx cbeaeb16a013161a98496fad62933b1d21786672
 clone git github.com/stretchr/testify 7c2b1e5640dcf2631213ca962d892bffa1e08860
 clone git github.com/tchap/go-patricia v2.1.0
 clone git golang.org/x/net 3cffabab72adf04f8e3b01c5baf775361837b5fe https://github.com/golang/net.git