Deploy fails when using secrets in task definition

[ChristophP]

ECS has a feature where it is possible to populate env variables from the ssm parameter store.
The task definition the looks kinda like this. https://docs.aws.amazon.com/AmazonECS/latest/developerguide/specifying-sensitive-data.html

{
  "containerDefinitions": [{
    "secrets": [{
      "name": "environment_variable_name",
      "valueFrom": "arn:aws:ssm:region:aws_account_id:parameter/parameter_name"
    }]
  }]
}

In order for secrets to work the property "executionRoleArn" must be set in the task definition but unfortately that property is not preserved by this script.

I opened a pull request to fix this. Would be great if it could be merged in since our deployments rely on this package. Thanks for making it <3

[jpcbarros]

+1 please merge the PR as I need this too :)