[file configuration] Clarify environment variable substitution is not recursive.

[marcalff]

The spec for environment variable substitution, in file_configuration, should clarify that substitution is not recursive.

For example:

export PING='${PONG}'
export PONG='${PING}'

some_yaml_node: ${PING}

should evaluate as string ${PONG}, and not call getenv() on PONG.

[jack-berg]

I agree. Would you care to open a PR?