Allow loading overrides for module defaults
    
    This patch adds a new config option. This is expected to be a name of
    subdirectory in the repo with module defaults. If supplied, overrides
    from that location are loaded every time defaults are loaded.
    
    This raises the minimal required version of libmodulemd to 2.8.0
    
    JIRA: COMPOSE-3828
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>