placeholder variable support

[flozano]

If using variables in some places with specific narrow validations (file sizes, boolean true/false values...), I get:

Multiple annotations found at this line:
	- cvc-pattern-valid: Value '${logback.debug.http.max-file-size}' is not facet-valid with respect to pattern '[1-9][0-9]*(KB|MB|GB)?' for type 
	 'FileSize'.
	- cvc-type.3.1.3: The value '${logback.debug.http.max-file-size}' of element 'maxFileSize' is not valid.

basically any element should be able to contain either a valid value or a placeholder expression (anything between ${ }). In pure string elements this already works as ${..} is a valid string, but in more strict elements the validation fails.