Parameterize admin and control container URIs on region

[tjkirch]

Builds on #636 and #637.

Fixes #231.

Testing done:

See #637. This is a separate PR to facilitate any potential discussion around actually enabling the feature, versus adding the capability, but this had to be there for testing #637.

[zmrow]

🌊

[tjkirch]

This push rebases onto #637 to get the typo fix caught by zmrow.

[bcressey]

My only nit here is the use of settings.aws.region in a global defaults.toml but we're planning to refactor that to support variants, right?

[tjkirch]

At the moment, every variant is aws, so this default is in the common location; we'd move it out to aws-specific variants as soon as we have something else. It might help if we have an "include" feature, or something similar, but we don't have to decide on that quite yet.

[tjkirch]

This push just rebases on the changes from #637.

[tjkirch]

This push rebases onto the new template syntax from #637. I apologize for the rebase being squashed with the changes; the only changes were in defaults.toml, where you now see just a "template" key and no longer the "parameters" key.

[zmrow]

🌡

[tjkirch]

This push just rebases on a typo fix from #636.

[tjkirch]

This push is just a rebase onto changes from #637.

[tjkirch]

Just a rebase on develop because of #637, no changes.