unify repo and repo_from options
    
    Config option 'repo' and 'repo_from' are used in several phases, merge
    them with one option 'repo'. 'append' in schema is used for appending
    the values from deprecated options to 'repo', so it won't break on any
    existing config files that have the old options of 'repo_from' and
    'source_repo_from' (which is an alias of 'repo_from').
    
    And 'repo' schema is updated to support repo dict as the value or an
    item in the values, a repo dict is just a dict contains repo options,
    'baseurl' is required in the dict, like:
    
    {"baseurl": "http://example.com/url/to/repo"}
    
    or:
    
    {"baseurl": "Serer"}
    
    currently this is used in ostree phase to support extra repo options
    like:
    
    {"baseurl": "Server", "exclude": "systemd-container"}
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>