Ensure all phases are stopped
    
    If a phase is started successfully, it needs to be stopped as well. In
    most cases when `stop` is called immediately after `start`, this is not
    a problem.
    
    Only when something else happens while a phase is runnning and this
    something fails, Pungi will deadlock and never exit. This something
    could be another phase or just main thread raising an exception.
    
    Fixes: #625
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>