Include the fedmsg-hub process.

Do this so that people can override the default ENTRYPOINT to use the
same container image to run the greenwave backend software.  See

- https://pagure.io/greenwave/pull-request/51
- https://pagure.io/greenwave/pull-request/86

Dockerfile

@@ -8,6 +8,7 @@ LABEL com.redhat.component="$NAME" \
       usage="docker run -p 8080:8080 -v greenwave-config:/etc/greenwave:ro greenwave" \
       url="https://pagure.io/greenwave"
 RUN dnf -y install --setopt=tsflags=nodocs \
+    fedmsg-hub \
     python-gunicorn \
     greenwave \
     && dnf -y clean all