Enable the solr s3 backup module by default in the solr service

The Upsun solr service ships with the s3 module installed, but it is not active, and cannot be configured given that we have no way to modify the base solr.xml file. 

Given that Upsun does not provide any means to backup large solr indexes, and given that there is a very capable and supported solr s3 module already installed in the service, it would be helpful if Upsun gave us the ability, while configuring the solr service, to set up s3 backups, so that we we can roll our own backups, since Upsun doesn't include solr data in their snapshots*.

(Upsun's docs say that solr backups are not necessary since the index can be re-created from underlying data. This is theoretically true, but in our case, it can take more than 10 days to index our data, and that is far too long for a recovery operation.) 

It would be a relatively minor change to the Upsun solr service to allow for a user to configure the s3 module for the solr service so that users can easily and in a supported fashion backup their solr data.