Use EnsureTask instead of prepending IG names to external ELB tasks

[rifelpet]

Fixes #10715

Partially reverts #10666

This way we end up with one CLB task per CLB regardless of how many ASGs it is attached to. Less brittle than creating many CLB tasks with concatenated names and exceptions for known CLBs.

The Name -> LoadBalancerName shouldn't make a functional difference, just improving consistency with how we attach CLBs to ASGs being created vs updated.

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[rifelpet]

note for reviewers: EnsureTask returns an error if the second invocation provides a task that isn't DeepEqual:

kops/upup/pkg/fi/task.go

Lines 70 to 74 in 0aa9cf1

	// EnsureTask ensures that the specified task is configured.
	// It adds the task if it does not already exist.
	// If it does exist, it verifies that the existing task reflect.DeepEqual the new task,
	// if they are different an error is returned.
	func (c *ModelBuilderContext) EnsureTask(task Task) error {

here, the CLB tasks should always be equal if a CLB is in multiple IGs' externaLoadBalancers because the only task fields that are set use the LoadBalancerName and shared=true, so this should never return an error.

[hakman]

Didn't know about this. May be a good idea to simplify the NLB side as well someday.

[hakman]

/lgtm