New method Compose.transition
    
    A comopse state transits several times in backend from wait to done and other
    finish state. This patch introduce a new method Compose.transition to
    change a compose' state with reason instead of repeating lines of code
    to change state, reason, corresponding time and commit.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>