Add support for variables in bundle config

[shreyas-goenka]

Changes

This PR now allows you to define variables in the bundle config and set them in three ways

1. command line args
2. process environment variable
3. in the bundle config itself

Tests

manually, unit, and black box tests

[pietern]

Interpolation will not be trivial as it is here when variables are a struct. Think about having a short hand for variable interpolation through var.foo if it would otherwise be variables.foo.value without further treatment.

[mgyucht]

Don't have too much context but thanks for having me review!

[pietern]

This happens too late. As is, schema --help will still display it. Has to happen in the init function below.

[shreyas-goenka]

It does not work from inside the init function. My hypothesis is that you cannot hide persistent flags

In this case, I would suggest let's just let the --var flag be visible for the schema command
Here is what the help output looks like:

shreyas.goenka@THW32HFW6T bricks % bricks bundle schema --help
Generate JSON Schema for bundle configuration

Usage:
  bricks bundle schema [flags]

Flags:
  -h, --help             help for schema
      --only-docs        only generate descriptions for the schema
      --openapi string   path to a databricks openapi spec

Global Flags:
  -e, --environment string       bundle environment to use (if applicable)
      --log-file file            file to write logs to (default stderr)
      --log-format type          log output format (text or json) (default text)
      --log-level format         log level (default disabled)
  -o, --output type              output type: text or json (default text)
  -p, --profile string           ~/.databrickscfg profile
      --progress-format format   format for progress logs (append, inplace, json) (default default)
      --var strings              set values for variables defined in bundle config. Example: --var="foo=bar"

Arguments for skipping this:

1. We should other flags like --profile, --output and the --environment flag which are not relevant for the schema command as well
2. It's nested under the Global Flags section which is less of a problem than if it was visible under the Flags sections
3. schema might be the only bundle command for which the --var flag is not needed

Arguments against:

1. Hiding persistent flags from parents in child commands is something that we might have to figure out in the future as well

[pietern]

Nice work. Please TAL at remaining comments before merging.

[pietern]

Question: should we allow this if the configuration has already provided a value?

[shreyas-goenka]

This follows the semantics from how terraform handles variables. You can define a default value in the config, and override it from the command line flags / env vars / module input parameters

Do you have any specific concerns here?

[pietern]

The difference with TF is that here the values can be hardcoded in the configuration and overridden per environment, for example. We need to have rule that says what happens if you try to set (through env or command argument) a variable with a value that has already been set.

Addressed all comments