Provide a way to configure the default image pull secrets based on env. variables.

[juan131]

Is your feature request related to a problem? Please describe.

As it was explained by @coro at this response, now it's possible to define the default RabbitMQ image to use setting the DEFAULT_RABBITMQ_IMAGE environment variable. However, it's not possible to define the default pull secret to use to retrieve that image (this is necessary when consuming images from private registries)

Describe the solution you'd like
The operator should be able to obtain the secret(s) to use to retrieve images from a new env. variable (e.g. DEFAULT_IMAGE_PULL_SECRETS)

Additional context
This request follows-up the discussion initially started at #856

[github-actions]

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