adds a decommision_host script that performs the steps outlined in mana

adds a decommision_host script that performs the steps outlined in https://mana.mozilla.org/wiki/display/SYSADMIN/Server+Decommissioning+Checklist#ServerDecommissioningChecklist-Inventory

Just give it a hostname and a bug number

Sample run: https://gist.github.com/solarce/f27ddd28ac6b852ac8c4