provider/aws: ASGs can wait for ELB InService

[phinze]

This enhances the waiting strategy introduced in #1839 to wait for each
ASG instance to become InService in attached ELBs before continuing.

[pmoust]

Great timing @phinze , I was about to open an Issue for this!

[mitchellh]

LGTM!

[phinze]

@pmoust 😀

Worth highlighting that, in addition to adding "min_elb_capacity" to optionally wait for InService on attached ELBs, this also enhances the default ASG-level waiting to include LifecycleState, in addition to HealthStatus.

In practice, it seemed that HealthStatus was almost immediately "Healthy" while LifecycleState spends some time in "Pending" before going "InService".

Apparently, one can use lifecycle hooks to cause "Pending:Wait" to wait for manual intervention on the instance before continuing, but I couldn't find documentation on the definition of "Pending" vs "InService" in the absence of hooks.

[ghost]

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