Added support for multiple aws partitions which closes #43

[bebold-jhr]

Summary

It seemed like an easy fix to support the feature request made in #43

How did you test this change?

Currently I have neither access to gov cloud nor to one in china. So I couldn't test it.

Issue

#43

[theopolis]

make it so

[theopolis]

Here is the CI error message @bebold-jhr:

Executing command: scripts/ci_tests.sh
--> terraform-aws-agentless-scanning: terraform fmt check
variables.tf
--> terraform-aws-agentless-scanning: Running tests at examples/multi-account-multi-region-auto-snapshot
Initializing modules...
Reading environment variable exporting file contents.
Reading environment variable exporting file contents.
- lacework_aws_agentless_management_scanning_role in ../..
- lacework_aws_agentless_scanning_global in ../..
- lacework_aws_agentless_scanning_region_usw1 in ../..
- lacework_aws_agentless_scanning_region_usw2 in ../..
There are some problems with the configuration, described below.

The Terraform configuration must be valid before initialization so that
Terraform can determine which modules and providers need to be installed.
╷
│ Error: Invalid validation error message
│ 
│   on ../../variables.tf line 327, in variable "aws_partition":
│  327:     error_message = "The aws partition must be one of [aws, aws-us-gov, aws-cn]"
│ 
│ The validation error message must be at least one full sentence starting with an uppercase letter and ending with a period or question mark.
│ 
│ Your given message will be included as part of a larger Terraform error message, written as English prose. For broadly-shared modules we suggest using a similar writing style so that the overall result will be consistent.
╵

[theopolis]

Currently I have neither access to gov cloud nor to one in china. So I couldn't test it.

I am worried that this will not work how we intend. Based on my understanding we will need more work to be able to facilitate the cross-account access from the normal "aws" partition: https://docs.aws.amazon.com/govcloud-us/latest/UserGuide/govcloud-iam.html

I advise that we wait on merging this until end-to-end testing and validation is completed.

[bebold-jhr]

@theopolis I agree.
This should be done if someone can thoroughly test this using gov cloud accounts.
I close this PR, because it is not officially supported by lacework anyway.