Improve consistency of command functions

[errordeveloper]

Description

A follow up to #1116.

Checklist

• Code compiles correctly (i.e make build)
• All unit tests passing (i.e. make test)

[rndstr]

there is a lot of repetition going on, could we possibly move these into functions?

• eks.New() could be something like eks.NewWithRegion(…) (ctl, error)
• for config defaults and validation could add cmdutils.ResourceCmd#GetDefaultedAndValidatedConfig(…) (cfg, error)

(i'm sure you'll come up with better names)

[errordeveloper]

@rndstr thanks, indeed this is getting a little mad. I've added a method that wraps it up.

[cPu1]

If we pass a ClusterProvider as an argument to command handlers like doCreateCluster, instead of constructing it here, we can inject a ClusterProvider with fake AWS API clients for api.ClusterProvider. That'd let us unit-test command handlers.

[errordeveloper]

Indeed, but let's make that change separately. I'm just trying to take care of consistency here. I think that change will be easier to make now.

Actually, we could just make it happen inside of NewCtl, i.e. make it possible to set the provider implementation in ResourceCmd (which gets constructed before the handler function runs).

[cPu1]

Sounds good to do it later in a separate PR.

[errordeveloper]

This could be something like:

if rc.MockProvider != nil {
  return rc.MockProvider, nil
}

ctl := eks.New(rc.ProviderConfig, rc.ClusterConfig)

[errordeveloper]

But let's leave until next iteration, I also have a PR to rename rc to simply cmd, which I promised to Martina a long time ago.