This config option, available under each provider pool section, can
contain static key-value pairs that will be stored in ZooKeeper on
each Node znode. This will allow us to pass along abitrary data from
nodepool to any user of nodepool (specifically, zuul). Initially, this
will be used to pass along zone information to zuul executors.
Change-Id: I126d37a8c0a4f44dca59c11f76a583b9181ab653
David Shrewsbury