Add amazon_side_asn argument to aws_ec2_transit_gateway.default resource and corresponding variable

[BFarand]

what

• The new variable 'amazon_side_asn' was added to the module with a default value of 64512 (AWS default, preserving current behaviour).
• Variable 'amazon_side_asn' is now provided as an argument to the 'aws_ec2_transit_gateway.default' resource.
• Documentation updated accordingly.

why

• Allows the ASN of a transit gateway to be set, allowing the best practice of having multiple transit gateways used in the same deployment to have different ASNs (https://docs.aws.amazon.com/vpc/latest/tgw/tgw-best-design-practices.html)
• In cases where a user is importing their existing infrastructure so it is managed via terraform, specifying the existing ASN means that the existing transit gateway does not need to be destroyed and recreated

references

• closes #27

[hans-d]

/terratest

[jordanjennings]

@hans-d We are waiting on this PR as well and I see it has been closed without additional comments. Has this been implemented in another PR? What's the intention with closing this? We'd love to see this change be merged to expose this configuration option as we are also using a forked version and we'd prefer to migrate back upstream. Thank you!

[hans-d]

@jordanjennings The PR is very stale - and part of a big sweep closing out the very old/incomplete PRs across the big set of repos. Feel free to open a new PR with updated changes.