refactor: wait for load balancer readiness using a private field

[justinsb]

This approach is more explicit than looking at the names of the target
groups, and using a private field is simpler.

[justinsb]

Looks like a known flake kubernetes/kubernetes#122333

I was worried tht if this was the first test to run, it might be an error with the "wait for ready" logic. But it wasn't, it was about 8 minutes into the test run.

/retest

[hakman]

@justinsb Wouldn't it be simpler to just always wait for NLB?
It was an optimisation, but we can live without it.

[justinsb]

Wouldn't it be simpler to just always wait for NLB?
It was an optimisation, but we can live without it.

I'm fine with that if that's what we want to do. It does take a while, but I figure people are going to be waiting anyway...

The important end-goal here is to get rid of the TargetGroups field (notice they aren't really used once we split out listeners!), so we can version them based on the LoadBalancer version. I also wanted to experiment with the idea of private fields for internal options, they don't get diffed etc so they're a little easier to work with (no need to sync actual & expected, won't show up in dry-run output). Because they're private we do have to set them via a method but ... that seems like a good trade-off.

In this particular case though, I don't mind either way - as long as we stop using TargetGroups!

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