Support for escaping environment variable syntax

[mx-psi]

The current wording on the specification regarding environment variable substitution does not support escaping said syntax. If somebody wants to explicitly set a field to ${ENV}, they won't be able to do so.

In the Collector we support $${ENV} for this. It would be desirable to:

1. decide whether this feature is supported on the Configuration specification
2. define its behavior on the specification.

[pellared]

Related (supset?) to #3914

[jack-berg]

Closing as a duplicate of #3914. Adding the collector syntax to the convo in #3914.