provider/aws: Discover supported EC2 platforms (EC2 Classic/VPC)

[radeksimko]

This will make it possible/easier to make decisions based on supported EC2 platforms (VPC and/or EC2 Classic) in the context of any AWS resource which needs make such decisions.

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-vpc.html#differences-ec2-classic-vpc

This would help in #6387, #9744 and possibly in aws_instance, aws_db_instance (security_groups vs vpc_security_group_ids). I'm not 100% sure about the latter because we may actually need to allow security_groups for VPC accounts too, because that's how you reference SGs in the default VPCs.

make test TEST=./builtin/providers/aws TESTARGS='-run=TestGetSupportedEC2Platforms -v'
 2>/dev/null
==> Checking that code complies with gofmt requirements...
==> Checking AWS provider for unchecked errors...
==> NOTE: at this time we only look for uncheck errors in the AWS package
go generate $(go list ./... | grep -v /terraform/vendor/)
go test -i ./builtin/providers/aws || exit 1
echo ./builtin/providers/aws | \
		xargs -t -n4 go test -run=TestGetSupportedEC2Platforms -v -timeout=30s -parallel=4
=== RUN   TestGetSupportedEC2Platforms
--- PASS: TestGetSupportedEC2Platforms (0.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	0.031s

[grubernaut]

LGTM!

[radeksimko]

I won't mention this in Changelog as it has no direct impact on the user.

[ghost]

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