Repository list and sync commands

Uses the foreman-tasks-hammer plugin for showing and waiting for
async task (see https://github.com/iNecas/foreman-tasks/pull/20 for
more details).

Usage:

    # trigger syncing and show the progress till the task finishes
    hammer repository synchronize --id 1

    # trigger syncing and don't wait for the task to finish
    hammer repository synchronize --id 1 --async