testEnterCredential observed to fail due to offline mirror.

Since downloading tool metadata seems inessential for the test, leaving it disabled as is the default.
503 Service Unavailable for http://mirrors.jenkins-ci.org/updates/updates/hudson.tasks.Ant.AntInstaller.json?id=hudson.tasks.Ant.AntInstaller&version=1.557-SNAPSHOT%2520(private-03%252F24%252F2014%252014%253A15%2520GMT-jenkins)