Elytron ExternalSaslServerFactory returns EXTERNAL mechanism name when Sasl.POLICY_PASS_CREDENTIALS provided

The method org.wildfly.security.sasl.external.ExternalSaslServerFactory.getMechanismNames(Map<String, ?>) returns the "EXTERNAL" name also in cases where Sasl.POLICY_PASS_CREDENTIALS property is provided with "true" value.

Only mechanisms which actually pass credentials should be returned for such policy (unless the special WildFlySasl#MECHANISM_QUERY_ALL property is given).