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
...
```
Run now takes a context as well as a statebag. We'll assign the context
to the blank identifier to prevent namespace collisions. We'll let the
step authors opt-in to using the context.
`find . -iname "step_*.go" -exec gsed -i'' 's/func \(.*\)Run(/func \1Run(_ context.Context, /' {} \;`
Rich Cavanaugh
DanHam
nywilken
cove
Megan Marsh
Adrien Delorme
programmer04
Adrien Delorme
Matthew Hooker
Chris Bednarski
Clint Shryock