LdapUserBackend: disable "health check"

I see no point in checking this at every login. It could however be a
nice addition for our config backends and the setup wizard. I'd also
opt for completely removing this parameter - who wants to use this
method should explicitely call it.

library/Icinga/Authentication/Backend/LdapUserBackend.php

@@ -170,7 +170,7 @@ class LdapUserBackend extends UserBackend
      *
      * @throws  AuthenticationException     In case an error occured or the health check has failed
      */
-    public function authenticate(User $user, $password, $healthCheck = true)
+    public function authenticate(User $user, $password, $healthCheck = false)
     {
         if ($healthCheck) {
             try {