decom professor, add decommissioning.pp

I have done all the lifecycle steps up
to "End steps"-section

https://wikitech.wikimedia.org/wiki/Server_Lifecycle

asks for this as the next one, but it depends on decom or reuse

DNS removal is I90b68c4ae6d89f758863b5057f0d2e63108e5adb

and the next steps would be disabling switch port,
wiping disk, and deciding on decom or reclaim

therefore i'm handing it to cmjohnson at this point

Change-Id: I39f860227fac83efe483486cf5893945cb9af242