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