Adding export Init method for the exported discovery components

Since Swarm discovery is used by libnetwork without the swarm main(), it
is forced to use annonymous import in order to force-trigger the
unexported init() methods. Using annonymous import is highly discouraged
in a Non-Main and Non-Test packages.

Introducing exported Init() methods on these files so that libnetwork
can use Discovery as a library without having a main package.

Signed-off-by: Madhu Venugopal <madhu@docker.com>

discovery/file/file.go

@@ -16,6 +16,11 @@ type Discovery struct {
 }
 
 func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
 	discovery.Register("file", &Discovery{})
 }
 

discovery/kv/kv.go

@@ -21,6 +21,11 @@ type Discovery struct {
 }
 
 func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
 	discovery.Register("zk", &Discovery{backend: store.ZK})
 	discovery.Register("consul", &Discovery{backend: store.CONSUL})
 	discovery.Register("etcd", &Discovery{backend: store.ETCD})

discovery/nodes/nodes.go

@@ -13,6 +13,11 @@ type Discovery struct {
 }
 
 func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
 	discovery.Register("nodes", &Discovery{})
 }
 

discovery/token/token.go

@@ -24,6 +24,11 @@ type Discovery struct {
 }
 
 func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
 	discovery.Register("token", &Discovery{})
 }