include environment in default outputPath

We are testing concurrent deployments using threads. We are doing this locally, but I figured it might make sense as the default:

```js
outputPath: `tmp/deploy-dist-${environment}`
```

I started making a PR, but since the environment is dynamic and the defaults are defined as static, I couldn't figure out how to get it to work, especially with the EMBER_CLI_DEPLOY_REUSE_BUILD section. Also, how would you detect if someone already customised the outputPath before appending the environment?

Start by tracing how the default outputPath is defined and how the EMBER_CLI_DEPLOY_REUSE_BUILD section uses it. Determine how a dynamic environment-based default should interact with customised outputPath values and concurrent deployments; done means the behavior is specified and covered for both default and customised paths.

Written by the indexing model from the issue text.