script to monitor, clean up salt keys of deleted labs instances

relies on: novaenv.sh, python-novaclient, salt-master
caveats:   - on the host where it runs, expects novaenv
             and salt master to manage/refer to a single
             identical realm (both eqiad, both codfw, etc.)
           - does not account for mult-master/syndic setups

* reports on undeleted nova instances with no salt keys and/or
  undeleted nova instances known to salt but unresponsive
* cleans up salt keys of deleted instances

Change-Id: Ib7da07d7e3ce01019e14e507805ae746afbebe48