Move package extras dependency to allow to have a docker image of gwcelery implementimg the command flower and shell

The current version of the gwcelery docker image (!1414 (merged)) is missing the command: flower Web based tool for monitoring and administrating Celery clusters.

To have a k8s deployment of gwcelery we need the docker image should also provide the command:

gwcelery flower --address=127.0.0.1 --log-file-prefix=gwcelery-flower.log

and such that the command

gwcelery shell

use Ipython instead of python.