load related modules only when specific auth method is enabled

Since Cantas supports several authentication strategies now and in the
future, most of the time, not all of them are enabled, in such a case
just load related dependent module for each enabled authentication
strategy is flexible and make it possible that administrator has the
opportunity to only install needed dependencies.

In particular, if someone deploys Cantas somewhere such as OpenShift
where it's impossible to `yum install` for Kerberos authentication, this
change can handle this situation easily.

services/auth/strategies.js

@@ -18,10 +18,8 @@
 
   'use strict';
 
-  var krb5 = require('node-krb5');
   var LocalStrategy = require('passport-local').Strategy;
   var RemoteUserStrategy = require('./remoteUserStrategy');
-  var GoogleStrategy = require('passport-google-oauth').OAuth2Strategy;
   var settings = require('../../settings');
   var User = require('../../models/user');
   var utils = require('../utils');
@@ -66,6 +64,8 @@
   var CantasKerberosStrategy = new LocalStrategy(function(kerberosName, password, done) {
     // asynchronous verification, for performance concern.
     process.nextTick(function() {
+      var krb5 = require('node-krb5');
+
       // IMPORTANT! NEVER store or log password here. It violates infosec policy!
       var principal = utils.build_krb5_user_principal(kerberosName, settings.realm);
       var email = principal.toLowerCase();
@@ -126,6 +126,8 @@
 
   var CantasGoogleStrategy;
   if (settings.auth.google && settings.auth.google.clientID && settings.auth.google.clientSecret) {
+    var GoogleStrategy = require('passport-google-oauth').OAuth2Strategy;
+
     CantasGoogleStrategy = new GoogleStrategy({
       clientID: settings.auth.google.clientID,
       clientSecret: settings.auth.google.clientSecret,