LDAP: Check for authtok validity
    
    The default authtok type in the LDAP provider (unlike the new IPA and AD
    providers) is "password". This oddity dates back to when password was
    the only supported authtok type in the SSSD, so configuration specifying
    only the password and bind DN was valid.
    
    We need to check the authtok validity as well before attempting to use
    it.