350 improvement serverless test workshop UI configuration improvement

[wtromano-aws]

Issue #, if available: 350

Description of changes:

• The application retrieves the backend API endpoint given the stack name in an environment variable name BACKEND_STACK_NAME (or defaults to urs-backend)
• Removed the ECS container resources for the UI front end, as this functionality has moved into the workshop project.

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[brnkrygs]

Looks good. The only thing that comes to mind for me would be to add some logging around which endpoint the app chooses (either the cloudformation output or the default).