Allow migration script to assume custom role

[maxammann]

The IAM user that applies the terraform config in our setup does not directly have permission to run the migrate.sh script.

It lacks:

      "application-autoscaling:*",
      "ec2:*",
      "ecs:*", # RunTask
      "iam:PassRole",

It would be great if we could let the executing user assume a role instead of giving permission directly.

I envision that the migration config in this TF module would allow to set an assume_role which is assumed before executing the migrate.sh script.

[harrisonravazzolo]

Hey @maxammann - check out this PR we are testing. #31

[zayhanlon]

assigning to @BCTBB to provide a final test, review with @edwardsb and then merge