This should help reduce the bulk of the review site backups
* launchpadlib cache has ~650,000 files which we don't need to track
* review_site/tmp has ~50,000 files
* review_site/cache is about 9gb
* review_site/index is optional to backup, but a) it's very unlikley
to be useful in a full restore situation; we'd have to re-create
them and b) things seem to come and go under this directory during
the backup, causing it to exit with an error status.
Change-Id: If7009cfcd5a3a07c07108149772cc8c1873bf277
6bcfe05742