Make db:nuke delete Solr data dir using full path

While digging around in the buildfile,  I figured that since the `set-up` solr task was setting the Solr `solr.data.dir` property to `${aspace.data_directory}/solr_index`, it made sense to have `db:nuke` delete that directory using the same reference.

Also, it turns out that our description for the `db:nuke` task was still the same as `db:migrate`! :scream: