Improve Celery config print on start-up

When running ewoksserver without a CELERY field in the config, this is what appears:

CELERY:
Not configured (local workflow execution)

which is good ✅

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When running ewoksserver with an empty dict in the CELERY field in the config, this is what appears:

CELERY:
{}

which is not great ❌ . It does not tell the user that ewoksserver will try to execute workflows via CELERY (remote execution).

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It would be nice to have instead something like

CELERY is configured! Workflows will be executed remotely using the following config:
{}

And for local execution:

CELERY is not configured! Workflows will be executed locally.