Refactor into dual Rancher API/Metadata providers

Introduces Rancher's metadata service as an optional provider source for
Traefik, enabled by setting `rancher.MetadataService`.

The provider uses a long polling technique to watch the metadata service and
obtain near instantaneous updates. Alternatively it can be configured to poll
the metadata service every `rancher.RefreshSeconds` by setting
`rancher.MetadataPoll`.

The refactor splits API and metadata service code into separate source
files respectively, and specific configuration is deferred to
sub-structs.

Incorporates bugfix #1414

vendor/github.com/rancher/go-rancher-metadata/main.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"time"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/rancher/go-rancher-metadata/metadata"
+)
+
+const (
+	metadataUrl = "http://rancher-metadata/2015-12-19"
+)
+
+func main() {
+	m := metadata.NewClient(metadataUrl)
+
+	version := "init"
+
+	for {
+		newVersion, err := m.GetVersion()
+		if err != nil {
+			logrus.Errorf("Error reading metadata version: %v", err)
+		} else if version == newVersion {
+			logrus.Debug("No changes in metadata version")
+		} else {
+			logrus.Debugf("Metadata version has changed, oldVersion=[%s], newVersion=[%s]", version, newVersion)
+			version = newVersion
+		}
+		time.Sleep(5 * time.Second)
+	}
+}