Use longer timeout for root console switch after liveinst
    
    For some reason, in recent tests, switching to a console after
    live install completes is taking a long time, and tests are
    failing because we 'only' allow 10 seconds for the login prompt
    to appear. This seems to indicate some kind of performance bug,
    but we don't really want all liveinst tests to fail on in, this
    is not primarily a performance testing framework. So let's
    tweak the root_console / console_login bits a bit to allow a
    configurable timeout for the login prompt to appear, and use
    that to wait 30 secs instead of 10 in this case.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>