Add a warning when using file:// state store

[rifelpet]

As discussed during office hours today. This already requires users to opt-in so they should already know what they're doing, but just in case we print this warning.

KOPS_STATE_STORE=file://$(pwd)/state kops update cluster foo.k8s.local
W0117 16:44:05.081437   92040 factory.go:122] The local filesystem state store is not functional for running clusters
I0117 16:44:15.092092   92040 executor.go:103] Tasks: 0 done / 137 total; 55 can run
I0117 16:44:16.268684   92040 executor.go:103] Tasks: 55 done / 137 total; 33 can run
...

This warning is printed on all kops commands that utilize the state store (replace, update, etc.)

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[joshbranham]

Nice!
/lgtm