Validate the provided username and password in the password validation realm used by the web container login
 mechanism configured for the <code>ServletContext</code>.

 <p>
 This method returns without throwing a <code>ServletException</code> when the login mechanism configured for the
 <code>ServletContext</code> supports username password validation, and when, at the time of the call to login, the
 identity of the caller of the request had not been established (i.e, all of <code>getUserPrincipal</code>,
