Expose a safe/machine readable way to check if an environment is already bootstrapped

The quickstart plugin needs to know if the environment is already bootstrapped, so that it can reuse an existing environment when called multiple time. To do that, it just executes the bootstrap (or bootstrap --debug) command and, if the subproces exits with an error, it parses the output for "environment is already bootstrapped" or similar. This is weak because we are relying on an error message intended to be read by humans. We need a reliable/machine readable way to make that check: perhaps a --format for the bootstrap output/error?