GCE backend requires "StrictHostKeyChecking no" in user ssh config

[wmertens]

If you deploy GCE nodes without "StrictHostKeyChecking no" in your own ssh config, the deploy will ask if it should add the keys and fail to deploy the host keys (requiring killing/redeploying the VMs).

[oconnorr]

Presumably we actually don't want to disable the host key check, rather nixops ought to be managing the known host keys itself and not touching the user's known host key file.