Support multi stage variable substitution

[matthewrmshin]

Sometimes users may want to build a YAML configuration in different stages, so it is not always desirable to issue an error when a variable substitution fails. We should add an option to allow the system to leave the variable substitution syntax in place for specifc variables.

[matthewrmshin]

Note a current workaround is to use escape syntax \${NAME} in the first pass.