common: Avoid using require to read JSON file key.

`require` is not working well with relative paths.

lib/common/connection.js

@@ -18,8 +18,10 @@
 
 'use strict';
 
+var fs = require('fs');
 var GAPIToken = require('gapitoken');
 var req = require('request');
+var path = require('path');
 var pkg = require('../../package.json');
 var util = require('./util');
 
@@ -53,12 +55,9 @@ Token.prototype.isExpired = function() {
  * @param  {Object} opts Options.
  */
 function Connection(opts) {
-  var credentials = opts.keyFilename && require(opts.keyFilename) || {};
-  // client email for the service account
-  this.email = credentials.client_email;
-  // contains the contents of a pem file
-  this.privateKey = credentials.private_key;
+  this.opts = opts || {};
 
+  this.credentials = null;
   this.scopes = opts.scopes || [];
   this.token = null; // existing access token, if exists
 
@@ -92,7 +91,8 @@ Connection.prototype.connect = function(callback) {
  * @param  {Function} callback Callback function.
  */
 Connection.prototype.fetchToken = function(callback) {
-  if (!this.email || !this.privateKey) {
+  var that = this;
+  if (!this.opts.keyFilename) {
     // We should be on GCE, try to retrieve token from
     // the metadata from server.
     req({
@@ -112,9 +112,24 @@ Connection.prototype.fetchToken = function(callback) {
     });
     return;
   }
+  if (!this.credentials) {
+    // read key file for once and cache the contents.
+    fs.readFile(this.opts.keyFilename, function(err, data) {
+      if (err) {
+        callback(err); return;
+      }
+      that.credentials = JSON.parse(data);
+      that.fetchServiceAccountToken_(callback);
+    });
+    return;
+  }
+  this.fetchServiceAccountToken_(callback);
+};
+
+Connection.prototype.fetchServiceAccountToken_ = function(callback) {
   var gapi = new GAPIToken({
-    iss: this.email,
-    key: this.privateKey,
+    iss: this.credentials.client_email,
+    key: this.credentials.private_key,
     scope: this.scopes.join(' ')
   }, function(err) {
     if (err) {