Parallelise NAT gateway deletion

[shivi28]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:
Added parallelization during NAT gateway deletion

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2263

Special notes for your reviewer:

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

NAT gateways are now deleted in parallel, which should reduce cluster deletion time by >50% for multi-AZ clusters

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/test ?

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/priority important-longterm

[sedefsavas]

Minor comment.

[sedefsavas]

/lgtm

[sedefsavas]

We can use kerrors.NewAggregate method for combining errors:

func (s *Service) Delete(m *scope.MachineScope) error {
	var errs []error
	for i := int32(0); i < m.GetSecretCount(); i++ {
		if err := s.forceDeleteSecretEntry(fmt.Sprintf("%s/%d", m.GetSecretPrefix(), i)); err != nil {
			errs = append(errs, err)
		}
	}

	return kerrors.NewAggregate(errs)
}

[sedefsavas]

/lgtm

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/approve

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