As I was fixing up the AccTests to not depend on a single existing IAM
role (which this commit does), I noticed that without some sleeping that
the kinesis_firehose_delivery_stream would often come back with:
```
msg: Firehose is unable to assume role {{arn}}. Please check the role provided.
code: InvalidArgumentException
```
Similar to the strategy taken in aws_instance with IAM Instance Profile errors,
I dropped in a simple retry loop which seemed to take care of the issue. Seems
that the same permission propagation delays apply here too.
Ian Duffy