Render recipe variants using a CBC file

[msentissi]

Description
In certain cases, it can be useful to render all of the variants of a conda recipe from a conda_build_config.yaml file, replacing the JINJA variables with the appropriate values.

The request is to add functionality to perform this variant rendering to the parser.

[schuylermartin45]

@msentissi and I briefly talked offline about this. I think the easiest path forward is to allow the RecipeReader base class take in an optional table of additional variables that can be used in JINJA substitutions and have the CbcParser class provide that table. I suppose that table could also be provided when get_value() is invoked. That may be more flexible for future work, rather than passing in a reference to the CbcParser directly.

There may also need to be some improvements around the SelectorParser class to fully support the ability to distinguish between recipes run on different platforms.

[schuylermartin45]

I've started the integration of the CBC variables into the RecipeReader class on this branch: https://github.com/conda/conda-recipe-manager/tree/smartin_cbc_vars_in_recipe_parser

[schuylermartin45]

This ends up being a bigger task than I was initially anticipating. I forgot that the CBC file tends to use a lot of selectors to indicate version numbers. So each variable could have multiple values, depending on the context.