Add Flower alongside live environment Celery workers

[ryanjjung]

This is a draft PR which will ultimately add Celery workers to our live environments alongside Flower for worker visibility.

Specifically, it contains:

• Flower added to local dev configuration
• Added explicit CONTAINER_ROLE variable value: "api" to indicate the regular backend as opposed to Celery, Flower, or something else.
• Built new AutoscalingFargateCluster with Celery and Flower running in dev.
• Restructured the dev config so that the Appointment task and container definitions are more "DRY".
• Allow the Celery and Flower containers in dev to talk to the Redis cluster.

Relevant Tickets

• Deploy Celery workers for Appointment #1611
• Build new ECS clusters for celery/flower #1613
• Add Flower for queue visibility #1615