Solr configuration tunables

Add some tunables to the solr plugin, to play with the HTTP client when using the external Solr server.

@Primer42 @dallasmarlow @maddalab
Is this necessary? On plugin validation, we require that the url is present: https://github.com/tumblr/collins/blob/gabe-solr-configs/app/collins/solr/SolrConfig.scala#L37