Implement distribution methods to replace the testapi defaults.
    
    This PR adds implementations of distribution methods to make them
    usable on Fedora. It adds the following methods
    
    * ensure_installed (to install packages)
    * become_root (to switch to root account)
    * script_sudo (run script with sudo)
    * assert_script_sudo (run and assert a sudo script)
    
    It also adds a helper script to the utils.pm
    
    * make_serial_writable
    
    that makes the serial console writable for normal users
    and so enables to run commands that check their progress
    by sending messages to the serial console. Normally, they
    fail, because the messages will not be written their, so
    the checking mechanism will never see them.