opam: Define a initialize_opam_root parameter for opam loader
When running in production, the workers should expect the opam root directory to be present (externally maintained). The production task should do nothing (initialize_opam_root's default value is False).
When running with standalone loader, they should be instantiated with initialize_opam_root to True. They will create the opam root folder if not present so it can work out of the box (e.g. docker worker)
Related to T3590
Test Plan
tox
Migrated from D6340 (view on Phabricator)