Add an option to allow more flexibility whilst copying files.

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, as
a result they want to be able to mirror files in such a way that old or missing files
are deleted.

The default functionality remains the same (i.e. no-op), adding a JSON array to the
copyRoots key in data will list multiple source roots from the package artifact directory
that should be copied to the target webapp root.

Similarly copyMode defaults to additive (the existing mode) but can be set to mirror which
in essence adds '--delete --delete-after' to the rsync command.
Merge pull request #108 from guardian/sh-copy-roots-and-mode

Add an option to allow more flexibility whilst copying files.