Re-consider instance naming pattern

[lukehoban]

We've adopted a pattern of using the name instance to refer to the raw resource that is morally the key thing wrapped by a higher level component in this library - like awsinfra.ecs.Cluster#instance will return the aws.ecs.Cluster.

This was done to avoid stuttering like cluster.cluster in common cases.

However, I would suggest that it is better to go with the cluster.cluster approach for a variety of reasons. It's not perfect, but I believe on balance it's the best option.

1. Typically this is just one of several underlying resources being managed, and although its the “main” one, it’s awkward to name it differently. Everything else is securityGroup, or subnet, but then the cluster is instance.
2. We've consistently seen everyone initially go with the cluster.cluster style naming by default, since it is the most natural option - and I think anything else is likely to not be adopted broadly enough.
3. The repetition just isn't that bad, and actually has a level of simple clarity to it - it's likely to be more discoverable and usable than alternatives.

[CyrusNajmabadi]

@lukehoban Sounds good. I've added this to the list in #53 and i'm closing this out.