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/metadata/change.go

@@ -0,0 +1,64 @@
+package metadata
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/Sirupsen/logrus"
+)
+
+func (m *client) OnChangeWithError(intervalSeconds int, do func(string)) error {
+	return m.onChangeFromVersionWithError("init", intervalSeconds, do)
+}
+
+func (m *client) onChangeFromVersionWithError(version string, intervalSeconds int, do func(string)) error {
+	for {
+		newVersion, err := m.waitVersion(intervalSeconds, version)
+		if err != nil {
+			return err
+		} else if version == newVersion {
+			logrus.Debug("No changes in metadata version")
+		} else {
+			logrus.Debugf("Metadata Version has been changed. Old version: %s. New version: %s.", version, newVersion)
+			version = newVersion
+			do(newVersion)
+		}
+	}
+
+	return nil
+}
+
+func (m *client) OnChange(intervalSeconds int, do func(string)) {
+	version := "init"
+	updateVersionAndDo := func(v string) {
+		version = v
+		do(version)
+	}
+	interval := time.Duration(intervalSeconds)
+	for {
+		if err := m.onChangeFromVersionWithError(version, intervalSeconds, updateVersionAndDo); err != nil {
+			logrus.Errorf("Error reading metadata version: %v", err)
+		}
+		time.Sleep(interval * time.Second)
+	}
+}
+
+type timeout interface {
+	Timeout() bool
+}
+
+func (m *client) waitVersion(maxWait int, version string) (string, error) {
+	for {
+		resp, err := m.SendRequest(fmt.Sprintf("/version?wait=true&value=%s&maxWait=%d", version, maxWait))
+		if err != nil {
+			t, ok := err.(timeout)
+			if ok && t.Timeout() {
+				continue
+			}
+			return "", err
+		}
+		err = json.Unmarshal(resp, &version)
+		return version, err
+	}
+}