feat: support govcloud and china regions

[jthomerson]

Rather than hard-coding the partition, pull it from the ARN

[jthomerson]

@jeremygiberson-at-privoro 👋

[jeremygiberson-at-privoro]

I'll need to do this during work hours, so I'll give it a try Monday.

[coveralls]

Coverage increased (+0.2%) to 90.385% when pulling 3cd95da on gov-cloud-partition into 793b516 on master.

[coveralls]

Coverage increased (+0.2%) to 90.385% when pulling 3cd95da on gov-cloud-partition into 793b516 on master.

[jeremygiberson-at-privoro]

FYI I didn't forget about this. I've got to wait for the next scheduled release before I can give this a try. GovCloud access is much more restricted so I can play around in it when ever I want.

Nov 18th is the next scheduled release that I should be able to try this out.

[jeremygiberson-at-privoro]

Finally had the opportunity to give this a test in Gov cloud.
Unfortunately, it did not work.

 Serverless Error ---------------------------------------
 
  Invalid parameter: TopicArn Reason: A us-gov-east-1 ARN must begin with arn:aws-us-gov, not arn:aws:sns:us-gov-east-1:#REDACTED#:canary-project-topic

It looks like the permission resource was not generated correctly:

    "HelloLambdaPermissionCanaryprojecttopic": {
      "Type": "AWS::Lambda::Permission",
      "Properties": {
        "FunctionName": {
          "Fn::GetAtt": [
            "HelloLambdaFunction",
            "Arn"
          ]
        },
        "Action": "lambda:InvokeFunction",
        "Principal": "sns.amazonaws.com",
        "SourceArn": {
          "Fn::Join": [
            ":",
            [
              "arn:aws:sns",
              {
                "Ref": "AWS::Region"
              },
              {
                "Ref": "AWS::AccountId"
              },
              "canary-project-topic"
            ]
          ]
        }
      }
    },

Here is my serverless.yml that I am using:

service: canary-project
frameworkVersion: '2'

plugins:
  - serverless-plugin-external-sns-events

provider:
  name: aws
  runtime: nodejs12.x

functions:
  hello:
    handler: handler.hello
    events:
      - externalSNS: 'canary-project-topic'
resources:
  Resources:
    CanaryProjectSnsTopic:
      Type: "AWS::SNS::Topic"
      Properties:
        DisplayName: Canary Project Topic
        TopicName: canary-project-topic