directory structure change to work with glide

glide has its own requirements. My previous workaround caused me some
code checkin errors. Need to fix this.

weed/operation/data_struts.go

@@ -0,0 +1,7 @@
+package operation
+
+type JoinResult struct {
+	VolumeSizeLimit uint64 `json:"VolumeSizeLimit,omitempty"`
+	SecretKey       string `json:"secretKey,omitempty"`
+	Error           string `json:"error,omitempty"`
+}