Adds ability for users to specify a custom ECSTaskExecutionRole (hype…

…rscience#3)

Adds ability for users to specify a custom ECSTaskExecutionRole to include additional IAM permissions (such as access to SSM secrets)

IAM Fixes (hyperscience#2)

Fixes IAM permissions related to var.task_role_arn

Changes type of var.extra_container_defs to 'any'

This way we can avoid the "all map elements must have the same type" error if we include multiple value types.

Adds taskDefinitionArn to task failure CloudWatch event rule