ostree-installer:  Allow multiple repos in ostree installer
    
    Add new key 'repo' to allow specifying multiple repos as the source
    repositories. And change 'source_repo_from' to allow specifying multiple
    vairant names to use variant repos as source repositories.
    
    Doc of 'source_repo_from' is updated to not mention URL is supported,
    though we still support that in code. User should add url of repos in
    'repo' key instead of 'source_repo_from'.
    
    Fixes: #508
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>