$VARNAME is what we use for environment variables, it's good to
separate.
Some people use envsubst for homemade variable interpolation, it's also
good to have separation there.
## Changes
New source of default values for variables - variable file
`.databricks/bundle/<target>/variable-overrides.json`
CLI tries to stat and read that file every time during variable
initialisation phase
<!-- Summary of your changes that are easy to understand -->
## Tests
Acceptance tests