Added tftp datasource for cloud config.

vendor/github.com/pin/tftp/backoff.go

@@ -0,0 +1,35 @@
+package tftp
+
+import (
+	"math/rand"
+	"time"
+)
+
+const (
+	defaultTimeout = 5 * time.Second
+	defaultRetries = 5
+)
+
+type backoffFunc func(int) time.Duration
+
+type backoff struct {
+	attempt int
+	handler backoffFunc
+}
+
+func (b *backoff) reset() {
+	b.attempt = 0
+}
+
+func (b *backoff) count() int {
+	return b.attempt
+}
+
+func (b *backoff) backoff() {
+	if b.handler == nil {
+		time.Sleep(time.Duration(rand.Int63n(int64(time.Second))))
+	} else {
+		time.Sleep(b.handler(b.attempt))
+	}
+	b.attempt++
+}