Support for multi-subnet fleets.

[bethac07]

[ErinWeisbart]

Depending on how your subnets are set up, you may have limitations on the number of machines you can use in the subnets because IPs are limited.
Would also simplify troubleshooting when the machine types you want are limited in availability.

Can probably fix by restructuring fleet file. (Make a dummy request in the console and copy that structure to our fleet file).

[ErinWeisbart]

Tried this config to get rid of subnet. Worked once. Second time gave me
spotFleetRequestConfigurationInvalid : m5.xlarge, ami-fad25980, Linux/UNIX: Security group sg-GROUP and subnet subnet-SUBNET belong to different networks.

{
"IamFleetRole": "arn:aws:iam::XXXXXXXXXXXX:role/aws-ec2-spot-fleet-role",
"AllocationStrategy": "lowestPrice",
"TerminateInstancesWithExpiration": true,
"LaunchSpecifications": [
        {
        "ImageId": "ami-fad25980",
        "KeyName": "KEYNAME",
        "IamInstanceProfile": {
                "Arn": "arn:aws:iam::XXXXXXXXXXXX:instance-profile/ecsInstanceRole"
        },
        "BlockDeviceMappings": [
        {
                        "DeviceName": "/dev/xvda",
                        "Ebs": {
                        "DeleteOnTermination": true,
                        "VolumeType": "gp2",
                        "VolumeSize": 8,
                        "SnapshotId": "snap-04007a196c0f3f398"
                }
                },
                {
                "DeviceName": "/dev/xvdcz",
                "Ebs": {
                        "DeleteOnTermination": true,
                        "VolumeType": "gp2"
                }
                }
        ],
        "SecurityGroups": [{"GroupId":"sg-ZZZZZZZZZ"}]
                }
        ],
"Type": "maintain"
}