Allow turning on issue tracking for only some namespaces
    
    This commit allows turning on issue tracking for only some namespaces
    specified in the configuration file.
    This way, on src.fp.o we can enable issue tracking in pagure for the
    tests namespace without enabling it for the rpms namespace (for example)
    since the issues for the rpms namespace are meant to be tracked in
    bugzilla.redhat.com
    
    Fixes https://pagure.io/pagure/issue/4244
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>