[BB-4779] Support theme overrides

Description

This PR adds support for using a custom theme with Open edX instances in Grove.

Apart from that, this introduces the following changes and improvements -

1. Replace the Ruby pipeline generation code with Python.
2. Ability to override sass variable using the trigger.
3. It now adds support for instance specific grove.yml config file that defines configurations for how to build custom images, install themes, etc.
4. Tools container image is now based on docker dind image to support building images.

Supporting information

https://tasks.opencraft.com/browse/BB-4779

Related template MR - grove-template!9 (merged)

Testing instructions

1. Check this pipeline that prepares theme, builds the custom image, and deploys in k8s cluster.
2. Check generated _lms_override.scss and common-variables.scss file in the s3 bucket
3. Check http://ac9bcd6554dc34b5bacf385c24528f57-634388063.us-east-1.elb.amazonaws.com/ to see template override in action.

Deadline

None

Other information

Reviewer

• @x29a