Adding GetRecommendedRdsInstanceType to RDS module

[alpacamybags118]

PR to address #783

This PR adds functionality to the RDS module to get a recommended instance type based on region, database engine, and a given list of instances. It will pick the first available one from the list or throw an error if none exist.

It uses the AWS SDK function DescribeOrderableDBInstanceOptions to achieve this.

I modeled it a lot off the EC2 get recommended instance type functionality, but it is somewhat simplified since the underlying AWS method used has less options.

[alpacamybags118]

Something that DescribeOrderableDBInstanceOptions exposes that I didn't think to add as an option is the DB engine-version. I believe there are cases where the version could make a difference on availability of an instance type, so i'll go ahead and add that. This code should still be reviewable as is, and I can make any feedback changes while I add that parameter.

[alpacamybags118]

Updated the code to also add engine version to the check

[brikis98]

This is great, thank you! Just a couple minor NITs in the comments, and I can kick off tests.

[alpacamybags118]

Done!

[brikis98]

Thanks! Kicking off tests now.

[brikis98]

Ha! Got a test failure related to the instance type for EC2 instances:

TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66: �[1m�[31mError: �[0m�[0m�[1mError launching source instance: Unsupported: The requested configuration is currently not supported. Please check the documentation for supported configurations.
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66: 	status code: 400, request id: 4e020e26-7c45-4ce8-a161-290cd71622d1�[0m
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66: 
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66: �[0m  on main.tf line 26, in resource "aws_instance" "example_public":
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66:   26: resource "aws_instance" "example_public" �[4m{�[0m
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66: �[0m
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z logger.go:66: �[0m�[0m
TestTerraformRemoteExecExample 2021-03-11T10:22:13Z retry.go:99: Returning due to fatal error: FatalError{Underlying: error while running command: exit status 1; �[31m

I think this is exactly the issue you're fixing for RDS here, but we need to use the GetRecommendedInstanceType for EC2 to solve it. Any chance you'd want to update that example/test accordingly?

The other test failure is unrelated; our GCP test account is hitting some quota issues that we're cleaning up separately.

[alpacamybags118]

Yup I can update that test

[alpacamybags118]

updated!

[brikis98]

Thank you!

One note: I think you may have accidentally checked in a .tfvars file in the most recent commit. Could you remove that?

[alpacamybags118]

Whoops, removed!

[brikis98]

OK, great, thank you. I'll kick off tests again.

[brikis98]

Tests passed. Merging now.

[brikis98]

https://github.com/gruntwork-io/terratest/releases/tag/v0.32.11