Set default host to solr, not localhost

Problem/Motivation

The default host is localhost in src/SolrConnector/SolrConnectorPluginBase.php even though DDEV works with solr, and solr is also used in the README.

Steps to reproduce

Try to create a new Solr server with the default settings, and fail.

Proposed resolution

Make the default settings work.

Remaining tasks

Set default host to be solr.