[CI] Allow for referencing multiple templates in a build configuration

Summary:
This will allow for easier factorization of common parameters between
builds.

Depends on D6951.

Test Plan: Run any CI build plan with or without a template list.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6952