Bugfixes (and small enhancements)

The grab solr task assume the downloaded file name is always `solr-{{solr_version}}`. For older version of apache solr, available on https://archive.apache.org/dist/lucene/solr, the file name is `solr-{{solr_version}}`. The first commit adds a `solr_basename` variable which can be used to customize the basename of the downloaded file.

In the startup script, the value of `SOLR_HOME` does not use the `solr_home` variable. The second commit fixes this.

The third and fourth commit are small enhancements.