BaseARN does not generate valid ARNs for GovCloud and CN partitions of AWS

[earlrob]

Gov Cloud and China regions use an alternate partition name for ARNs. The following docs and code from aws which show how to properly build a region-agnostic arn: aws arn docs
TL;DR
example arn

arn:partition:service:region:account-id:resourcetype:resource

partition : The partition that the resource is in. For standard AWS regions, the partition is aws. If you have resources in other partitions, the partition is aws-partitionname. For example, the partition for resources in the China (Beijing) region is aws-cn.

awscli
TL;DR

def _get_policy_arn_suffix(region):
  region_string = region.lower()
  if region_string.startswith("cn-"):
   return "aws-cn"
  elif region_string.startswith("us-gov"):
    return "aws-us-gov"
  else:
    return "aws"

[markpeek]

Fixed via merge of #85. Thanks!