Fix rpm command to get changelog from SPEC
    
    Commands.rpmdefines was not added to rpm command to get changelog from
    SPEC. This causes SPEC cannot be parsed correctly if it contains SourceN
    or PatchN. Like other methods in Commands class that runs rpm,
    Commands.rpmdefines should be always added to the rpm command.
    
    Resolves: rhbz#1412224
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>