improve validation for CeleryExecutor (ensure redis is enabled)

[thesuperzapper]

Currently, we validate that the user has disabled workers.enabled, flower.enabled, and redis.enabled if airflow.executor=KubernetesExecutor.

We should also validate the reverse for CeleryExecutor and CeleryKubernetesExecutor, that is we should ensure that:

• Either embedded or external Redis is enabled
• NOTE: we shouldn't force flower to be enabled, as not all users will want flower