Allow the possibility to specify CELERY_CONFIG_MODULE=...

We should add configuration value .Values.gwcelery.config to specify the value used as:

• CELERY_CONFIG_MODULE={{ .Values.gwcelery.config }}

Now it is define the value: gwcelery.celeryConfig that allows to pass the content of a configuration file (that will be used as CELERY_CONFIG_MODULE='gwcelery.conf.helm-k8s'). and if this is not specified, it will be used CELERY_CONFIG_MODULE='gwcelery.conf.minikube'.