resource/aws_spot_fleet_request: Launch Template Support (hashicorp#1…

…2732)

Output from acceptance testing:

```
--- PASS: TestAccAWSSpotFleetRequest_associatePublicIpAddress (416.10s)
--- PASS: TestAccAWSSpotFleetRequest_basic (346.62s)
--- PASS: TestAccAWSSpotFleetRequest_changePriceForcesNewRequest (612.73s)
--- PASS: TestAccAWSSpotFleetRequest_disappears (229.25s)
--- PASS: TestAccAWSSpotFleetRequest_diversifiedAllocation (351.50s)
--- PASS: TestAccAWSSpotFleetRequest_fleetType (286.87s)
--- PASS: TestAccAWSSpotFleetRequest_iamInstanceProfileArn (285.22s)
--- PASS: TestAccAWSSpotFleetRequest_instanceInterruptionBehavior (284.16s)
--- PASS: TestAccAWSSpotFleetRequest_LaunchSpecification_EbsBlockDevice_KmsKeyId (142.74s)
--- PASS: TestAccAWSSpotFleetRequest_LaunchSpecification_RootBlockDevice_KmsKeyId (145.99s)
--- PASS: TestAccAWSSpotFleetRequest_launchSpecToLaunchTemplate (540.97s)
--- PASS: TestAccAWSSpotFleetRequest_launchTemplate (284.34s)
--- PASS: TestAccAWSSpotFleetRequest_launchTemplate_multiple (213.22s)
--- PASS: TestAccAWSSpotFleetRequest_launchTemplateToLaunchSpec (542.07s)
--- PASS: TestAccAWSSpotFleetRequest_launchTemplateWithOverrides (216.46s)
--- PASS: TestAccAWSSpotFleetRequest_lowestPriceAzInGivenList (273.30s)
--- PASS: TestAccAWSSpotFleetRequest_lowestPriceAzOrSubnetInRegion (345.74s)
--- PASS: TestAccAWSSpotFleetRequest_lowestPriceSubnetInGivenList (285.52s)
--- PASS: TestAccAWSSpotFleetRequest_multipleInstancePools (351.19s)
--- PASS: TestAccAWSSpotFleetRequest_multipleInstanceTypesInSameAz (337.09s)
--- PASS: TestAccAWSSpotFleetRequest_multipleInstanceTypesInSameSubnet (216.98s)
--- PASS: TestAccAWSSpotFleetRequest_overriddingSpotPrice (320.64s)
--- PASS: TestAccAWSSpotFleetRequest_placementTenancyAndGroup (63.07s)
--- PASS: TestAccAWSSpotFleetRequest_tags (354.51s)
--- PASS: TestAccAWSSpotFleetRequest_updateExcessCapacityTerminationPolicy (542.49s)
--- PASS: TestAccAWSSpotFleetRequest_updateTargetCapacity (828.53s)
--- PASS: TestAccAWSSpotFleetRequest_withEBSDisk (279.53s)
--- PASS: TestAccAWSSpotFleetRequest_WithELBs (268.50s)
--- PASS: TestAccAWSSpotFleetRequest_withoutSpotPrice (350.86s)
--- PASS: TestAccAWSSpotFleetRequest_withTags (282.52s)
--- PASS: TestAccAWSSpotFleetRequest_WithTargetGroups (410.53s)
--- PASS: TestAccAWSSpotFleetRequest_withWeightedCapacity (289.97s)
```