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/utils.go

@@ -0,0 +1,19 @@
+package metadata
+
+import (
+	"time"
+)
+
+func testConnection(mdClient Client) error {
+	var err error
+	maxTime := 20 * time.Second
+
+	for i := 1 * time.Second; i < maxTime; i *= time.Duration(2) {
+		if _, err = mdClient.GetVersion(); err != nil {
+			time.Sleep(i)
+		} else {
+			return nil
+		}
+	}
+	return err
+}