Make some HttpServer2 SSL properties optional

HttpServer2#loadSSLConfiguration loads 5 SSL properties but only keystore location and password are required, the rest of them, keystore keypassword, truststore location, and truststore password, can be optional.

According to http://www.eclipse.org/jetty/documentation/current/configuring-ssl.html:

• If there is no keymanagerpassword, then the keystorepassword is used instead.
• Trust store is typically set to the same path as the keystore.