Add an option to allow files to be deleted during deploy

Discussion would like to have this functionality as they have had a deploy failure mode
caused by old solr config files that had previously been deployed but no longer are.

The default functionality remains the same (i.e. no-op), adding a JSON array to the
predeployDeleteFiles key in data will delete the specified list of files.
Merge pull request #107 from guardian/sh-add-delete-files-option

Add an option to allow files to be deleted during deploy