Replace mainline depot with branchline depot in conductor.py

[ayushuk]

Branchline templates will be hosted on a new repo pros-branchline which stores kernel, okapi and branchline template versions. This is separate from pros-mainline.

At the moment conductor.py only supports pros-mainline and needs to be changed to get templates from branchline exclusively. This includes modifying conductor logic to support the new metadata and JSON format for template versions. resolve_template and fetch_template is out of scope.

Refer to the branchline design doc for more information.