mediawiki: enhancements to hhvm_cleanup_cache

* Import some of the additions in
  Ib2a0401a7aa5632fb79a5b17c0d0cef8955cf990
* Move to /usr/local/sbin
* Move to mediawiki::hhvm::housekeeping, which is probably the best
  place for this script

Change-Id: Ibcad98a95413044fd6c5e9bd3c0a6fb486bd5fe9