Get rid of implicit message topic prefix
    
    Callers of simple_message_to_bus need to set and export MSGTOPIC_PREFIX
    explicitly.
    
    This decouples the fedora-messaging-utils and web-data-analysis roles.
    
    Additionally, don't assume /bin/sh is /bin/bash.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>