Create a Registry of variables to be used from operation expressions.

See https://jira.nuxeo.org/browse/SUPNXS-17.

A variable registry should be exposed in operation expressions. This way variable that can be defined for each target platform
can be used in operation expressions.

To define a variable you simply create a ja a.properties file in nuxeo config directory and put there you variable definitions:
my.var=the value

Then from the operation you can access the variable using the "Env" map:

expr: Env["my.var"]

Note that all the nuxeo environment variables are accessible from the Env map - so you need to correctly prefix your variable name to avoid name clashes with existing variables.