Add functionality to add old buckets to etcd on startup

Buckets already present on a Minio server before it joins a
bucket federated deployment will now be added to etcd during
startup. In case of a bucket name collision, admin is informed
via Minio server console message.

Added configuration migration for configuration stored in etcd
backend.

Also, environment variables are updated and ListBucket path style
request is no longer forwarded.

cmd/routers.go

@@ -49,8 +49,7 @@ func registerDistXLRouters(router *mux.Router, endpoints EndpointList) {
 var globalHandlers = []HandlerFunc{
 	// set HTTP security headers such as Content-Security-Policy.
 	addSecurityHeaders,
-	// set Bucket forwarding handler to proxy path style requests
-	// when federated backend is enabled.
+	// Forward path style requests to actual host in a bucket federated setup.
 	setBucketForwardingHandler,
 	// Ratelimit the incoming requests using a token bucket algorithm
 	setRateLimitHandler,