InstanceRequirements is very flakey, the create fleet request fails
almost consistently with the same error.
To continue with testing use a fixed instance type for now. As a
followup we can expand the instance type selection logic or figure out
what was wrong with the InstanceRequirements.
For non-default VPCs, AWS needs the subnets it can launch the instance
in, otherwise it will try to launch the instance in the default VPC,
even if the supplied security groups are attached to a non-default VPC.
Furthermore there can only be 1 subnet specified per availability zone,
so query the subnets in the VPC of the host (as the instance needs to be
launched in the same network), and pick 1 of the VPC's subnets per AZ.
