pkgset: Add ability to wait for signed packages
    
    If packages are appearing quickly in Koji, and signing them is triggered
    by automation, there may be a delay between the package being signed and
    compose running. In such case it may be preferable to wait for the
    signed copy rather than fail the compose.
    
    JIRA: RHELCMP-3932
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>