Support for NGINX Variables

[oliveromahony]

There are a clear set of supported variables in NGINX, documented here: http://nginx.org/en/docs/varindex.html
It would be good for crossplane to support these variables and have an option to set real values in the NGINX config for the in-memory representation of the NGINX config. This will help users of crossplane to toggle between variables e.g. $hostname and actual values e.g. www

Describe the solution you'd like

Add a configuration that allows crossplane to change variables like $var into real values.

[ornj]

Can you elaborate on this idea and what problems it helps to solve? What is the expected behavior, perhaps with some example use cases / scenarios? Do you provide think Crossplane supports different values for variables that appear in multiple contexts? How are variables provided? Is it a dictionary of sorts? Is Crossplane processing something that looks like a request to set arbitrary variables such as $arg_ and $http_ or chose values from map directives?