CloudFormation template: ECS cluster creation can fail if not dependent on ExecutionRole

[ag-adampike]

When using our CloudFormation template, ECS cluster creation may fail if the ExecutionRole resource has not been created. Example error:

Invalid request provided: CreateCluster Invalid Request: Unable to assume the service linked role. Please verify that the ECS service linked role exists.

When encountering this, the error was corrected by making the ECSCluster dependent on the ExecutionRole.