Stop requiring jetty.xml edits to enable bin/solr to start in SSL mode

Right now we tell people to edit server/etc/jetty.xml to enable SSL: comment out the non-SSL connector(s), uncomment the SSL connector.

Jetty can be started using alternate configuration files - see https://wiki.eclipse.org/Jetty/Reference/jetty.xml_usage - we should make use of this capability and provide an SSL-enabled alternative to jetty.xml that bin/solr start can use when SSL is enabled. That way no manual edits to jetty.xml will be required.