Create new Runroot class and use it everywhere where runroot task is executed.
    
    This adds new `Runroot` class and new `runroot_method` option which makes
    it possible to choose between two currently available runroot methods:
    
    - Local
    - Koji
    
    The main goal of this commit is to make it possible to add new runroot
    methods in the future and this is the first step in that direction.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>