Add support for Direct Connect Gateway

[ewbankkit]

Fixes #2140.
Replaces #2861.

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAwsDxGateway_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -run=TestAccAwsDxGateway_ -timeout 120m
=== RUN   TestAccAwsDxGateway_importBasic
--- PASS: TestAccAwsDxGateway_importBasic (48.28s)
=== RUN   TestAccAwsDxGateway_importComplex
--- PASS: TestAccAwsDxGateway_importComplex (1289.32s)
=== RUN   TestAccAwsDxGateway_basic
--- PASS: TestAccAwsDxGateway_basic (43.90s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	1395.400s

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAwsDxGatewayAssociation_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -run=TestAccAwsDxGatewayAssociation_ -timeout 120m
=== RUN   TestAccAwsDxGatewayAssociation_basic
--- PASS: TestAccAwsDxGatewayAssociation_basic (1001.33s)
=== RUN   TestAccAwsDxGatewayAssociation_multiVgws
--- PASS: TestAccAwsDxGatewayAssociation_multiVgws (1075.20s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	2089.566s

[ewbankkit]

This PR enhances #2861:

• Custom RandIntRange function from helper/acctest: Fix RandIntRange function terraform#17438 so acceptance tests pass
• Rename 'virtual_gateway_id' attribute to 'vpn_gateway_id' as discussed here
• Add timeout configuration for aws_dx_gateway and aws_dx_gateway_assocation resources
• Wait for the aws_dx_gateway_assocation resource to reach the associated state when creating and the disassociated or deleted state when destroying
• Better error handling for already deleted gateways
• Incorporate fix from r/aws_vpn_gateway_attachment: Correct error code for missing VPN gateway #4895 for acceptance tests

[ewbankkit]

Once this is merged we can add some test cases with Direct Connect gateways to #3255 and #3253.

[ewbankkit]

Rebased to remove the merge error.

[bflad]

Duplicate return nil here causing the TravisCI failure 😉

[bflad]

@aeriff and @ewbankkit 🥇 🥇 🥇 Thank you so much for your perseverance here. Let's ship this initial implementation as-is with the combined commits -- it looks pretty good to me and passes acceptance testing. We can tease out any fixes/improvements in future releases. 🚀

[bflad]

This has been released in version 1.24.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!