Use versioned config file in render/start as replacement of flags

[aaronlevy]

Before going 1.0 bootkube should have some compatibility enforcement. It's difficult to do this with flags, so we probably should move to something like a versioned config file that is consumed by both the render and start subcommands.

[colemickens]

/area render

[colemickens]

start currently only takes two flags: --asset-dir and --pod-manifest-path.

I would suggest that we treat --asset-dir as the pointer to the state+configuration for the cluster. Any configuration file would be implicitly part of the asset dir. In the asset dir, we could introduce a new configuration file bootkube.yaml or some such, with cluster-specific details, like the kubelet pod manifest path location.

We could continue to support the flags for some time, persisting them into the asset-dir as we render, and encourage users to move to a configuration file and then eventually drop the individual flags we support in render / start other than --asset-dir.

Alternatively, we might consider adopting either kubeadm's "MasterConfiguration" yaml config file and supporting a subset of fields, or probably better, support the new Cluster API which seems built for tools like bootkube.

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