templating for test manifests

[edaniszewski]

Right now there are two somewhat inconvenient ways of approaching tests that use similar, but different, manifests (e.g. only a few values are changed).

1. Make separate copies for each test
2. Make a basic manifest and then modify it once it has been loaded

Option 2 prevents you from using the applymanifest marker, since you would need to manually update the config.

To improve this flow, we could support templating (e.g. some kind of simple jinja) for the manifests. tbd how it would work with the applymanifests marker, but worst case is we could just pass a dict of values to the marker for templating.