So that it works for all builders as some use it with the following field names: launch_block_device_mappings, ami_block_device_mappings or ebs_volumes. the refactored docs weird being to specific.
Subnet information is only really needed when the specified `vpc_id` is
not the default VPC for the region where the builder is being executed.
This change uses the AWS API to determine if the VPC provided is a
non-default VPC and only validates the existence of a `subnet_id` if a
user has provided a non-default `vpc_id`.
Tests after change
```
> make test TEST=./builder/amazon/... TESTARGS='-count=1 -v -run=TestStepPreValidate_checkVpc'
...
=== RUN TestStepPreValidate_checkVpc
=== RUN TestStepPreValidate_checkVpc/DefaultVpc
=== RUN TestStepPreValidate_checkVpc/NonDefaultVpcNoSubnet
=== RUN TestStepPreValidate_checkVpc/NonDefaultVpcWithSubnet
=== RUN TestStepPreValidate_checkVpc/SubnetWithNoVpc
=== RUN TestStepPreValidate_checkVpc/NoVpcInformation
--- PASS: TestStepPreValidate_checkVpc (0.00s)
--- PASS: TestStepPreValidate_checkVpc/DefaultVpc (0.00s)
--- PASS: TestStepPreValidate_checkVpc/NonDefaultVpcNoSubnet (0.00s)
--- PASS: TestStepPreValidate_checkVpc/NonDefaultVpcWithSubnet (0.00s)
--- PASS: TestStepPreValidate_checkVpc/SubnetWithNoVpc (0.00s)
--- PASS: TestStepPreValidate_checkVpc/NoVpcInformation (0.00s)
PASS
...
```
Adam Dobrawy
Megan Marsh
Lars Lehtonen
Ilhaan Rasheed
Moss
Jon Allie
Aurélien Guillaume
Mike Dziedziela
henry-aj-jones
Anthony Dong
DanHam
nywilken
