Configure AIrflow to use CeleryExecutor by default

[btylerburton]

User Story

In order to configure a more robust, distributed system, datagovteam wants to configure the Airflow instance in Cloud.gov to utilize the CeleryExecutor.

Related:

• Configure Production Image Setup using LocalExecutor #4511

Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using BDD.]

• GIVEN I run cf login -a api.fr.cloud.gov --sso and authenticate
  AND I run cf target -o gsa-datagov -s <development|staging|production>
  WHEN I run cf push airflow --vars-file my_vars_file --strategy rolling
  AND I visit the configuration at https://airflow-test.app.cloud.gov/configuration
  AND look up the core: execuctor setting
  THEN I will see it is using CeleryExecutor.

Background

This builds on our first CG implementation ticket by implementing a production ready system, with the capacity for true horizontal scaling.

The advantages of the CeleryExecutor over the LocalExecutor are that it allows for scaling across multiple application instances and is more fault tolerant.

Also, in contrast to the KubernetesExecutor it can live fully within the Cloud.gov ecosystem, which is beneficial for compliance purposes.

Security Considerations (required)

[Any security concerns that might be implicated in the change. "None" is OK, just be explicit here!]

Sketch

• Configure a cloud.gov Elasticache Redis instance
• Configure current manifest to use CeleryExecutor
• Deploy the application onto cloud.gov and allow it to process