Retry checking spot instance request state on AWS::EC2::Errors::InvalidSpotInstanceRequestID::NotFound

Creation of jobs based on AWS spot instances often fail with errors like the following:
```
D, [2014-05-10T08:16:45.392793 #3757] [create_vm(d8c85ece-3d74-43df-9c6d-2da7dd03f9aa, ...)] DEBUG -- : Checking state of spot instance requests...
I, [2014-05-10T08:16:45.481592 #3757] [create_vm(d8c85ece-3d74-43df-9c6d-2da7dd03f9aa, ...)]  INFO -- : [AWS Core 400 0.088475 0 retries] describe_spot_instance_requests(:spot_instance_request_ids=>["sir-37a31a3b"]) AWS::EC2::Errors::InvalidSpotInstanceRequestID::NotFound The spot instance request ID 'sir-37a31a3b' does not exist
E, [2014-05-10T08:17:45.482022 #3757] [create_vm(d8c85ece-3d74-43df-9c6d-2da7dd03f9aa, ...)] ERROR -- : The spot instance request ID 'sir-37a31a3b' does not exist (AWS::EC2::Errors::InvalidSpotInstanceRequestID::NotFound)
```

This is caused by a delay in AWS updating its state - the spot request has been created, but isn't yet available for querying via `describe_spot_instance_requests`

Retrying the `describe_spot_instance_requests` resolves the error; which is what this PR implements