stage0/run: relax '--hosts-entry' parser

[lucab]

This tweaks the '--hosts-entry' CLI flag parser to accept multiple
time the special value "host" when there are no other conflicting
settings.

[lucab]

/cc @squeed

[lucab]

@euank I think CL may need this to go together with the kubelet-wrapper change, would you mind a review? See poseidon/matchbox#662 for context.

/cc @dghubble

[dghubble]

This would be a welcome addition to avoid breaking any users who already set the flag directly. 👍

rel: coreos/coreos-overlay#2839, kubernetes-retired/bootkube#740

[euank]

I guess now if someone noticed this before and fixed it with the alternate --host-entry "127.0.0.1=localhost" it will fail and there's no particularly good way for the user to override this without forking the whole script.

I think this is probably the best approach we have still in the face of reality.

[lucab]

@euank right, this can't help resolving ambiguous inputs. It just relaxes the multiple-special-values case.