Make --flavor, --flavor-version, --version flags behave consistently between `fetch` and `switch` about getmesh

[ZackButcher]

Possibly instead of getistio switch <version name> we could do getistio switch --name 1.9.0-istio-v0 (if we want to preserve the ability to add subcommands to switch later).

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Forgot to add to OP, but ideally getistio switch --version 1.8.3 would work too, using the default flavor and flavor-version (so would swap to 1.8.3-tetrate-v0). Even better would be to swap version but keeping flavor that I have selected: so if my active is 1.8.3-istio-v0 and I getistio switch --version 1.9 it would assume --flavor istio because active command flavor = istio.

And ideally the same would work with just --flavor-version too: getistio switch --flavor-version 1 would use current flavor/version but swap flavor-version.

So end result would be that I can set any of --flavor, --version, --flavor-version and the rest will just "do the right thing" (= keep the others the same as the current active, or choose latest as default if there is no active).