Unbreak AWS cluster naming

[moio]

The AWS backend got inadvertently broken by #19

go code expects specific cluster names: upstream, tester, and downstream + a suffix

Current AWS code returns names like upstream-0 or tester-1 which result in dartboard deploy failing.

This PR removes the ability to set a prefix and adds suffix numbers only to downstream to keep the contract with the go part of the code.

Tested manually on AWS.

[git-ival]

I'm not sure this was necessarily needed since the defaults still made the module functional. I would have preferred if we didn't need to rely on explicitly naming clusters based on where they are in the "stream", but LGTM.

[moio]

Well in my tests defaults were actually broken, I can imagine there were other differences between my setup and yours.

About ordering: if we have any reasons for distinguishing downstream clusters based on the template they were created from, I am not against either going back to downstream-$TEMPLATE_INDEX-$CLUSTER_INDEX or even downstream-$TEMPLATE_NAME-$CLUSTER_INDEX. It's just that I did not see a need right now.

Happy to complicate as soon as there is a need.

Thanks, merging for now, we can iterate further later.