kojiwrapper: Ignore warnings before task id
    
    When looking for task ID in output of koji runroot command, do not check
    just the first line. Instead look for first line that contains just a
    number.
    
    Most of the time, this should really be the first line. But if koji
    client decides to print any warnings, this patch should skip that.
    
    JIRA: RHELCMP-8944
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>