When a broker's storage is deleted, it comes back with a new broker id.
There's no indication as to why the data is gone or whether it can be
recovered. Since the Cluster resource is deprecated and the cloud team
would just like to have a method to delete it regardless, the consensus
is that we should assume it's not recoverable and decommission the old
broker id immediately.

This PR adds a hidden flag, `unsafe-decommission-failed-brokers` that,
if set to true, will offer this behavior.

This is dangerous and shouldn't be used. There are circumstances in which
brokers with valid data can be decommissioned and even the potential for
all the brokers to be decommissioned.