variants: Reject values with whitespace
    
    If there is leading or trailing whitespace in a comps group name, it
    will not be included in the compose and there will even be no error
    message. Whitespace on module name results in a failure.
    
    To avoid these errors, validating the variants file will now also check
    that there is no whitespace in significant places, and abort the compose
    if there a problem.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>