Following on #1567, which expanded the scope a bit.

Here's the delta:

• New required atc property, token_signing_key
• New required tsa property, token_signing_key
• New required groundcrew property, tsa.worker_key, type ssh
• Removed groundcrew property, tsa.private_key, which was just the private key portion
• Replaced tsa property, host_key, which is now of type ssh, containing both the public and private portion.
• Removed tsa property, host_public_key; superseded by the above property
• Removed tsa property, authorize_generated_worker_key; no longer means anything
• The tsa authorized_keys property must now be specified. No workers are automatically authorized anymore.

The docs will need to be updated for this. We should maybe even just start on the cluster/ portion of concourse-deployment and have the docs reference it, instead of inlining a manifest. (This isn't really implied by the necessary docs work, but we may want to just do it at the same time.)