Change external api for GenericBuilder class
    
    - merge buildroot_prep and buildroot_resume under buildroot_conennect https://pagure.io/fm-orchestrator/issue/34 (idempotent movement)
    - rename add_dependency to add_repo
    - provide workflow comparison koji vs copr as a docstring
    - update docstrings for GenericBuilder
    
    Signed-off-by: Lubos Kocman <lkocman@redhat.com>