Cross region SNS trigger
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# This is a (Bug Report)

## Description

When trying to deploy a lambda with an SNS trigger in a different region the cloudformation script fails.

* What went wrong?

Cloudformation fails to create SNS event subscription for lambda with error:

`invalid parameter: TopicArn`

* What did you expect should have happened?

Expected the lambda to deploy (via cloudformation) with a trigger on an SNS topic in a different region.

* What was the config you used?

```
service: testservice
provider:
  name: aws
  runtime: nodejs6.10
  region: us-west-1

functions:
  hello:
    handler: handler.run
    events: 
      - sns: arn:aws:sns:eu-west-1:xxxxxxxxxxx:stresstest
```

* What stacktrace or error message from your provider did you see?
```
Serverless: Packaging service...
Serverless: Uploading CloudFormation file to S3...
Serverless: Uploading artifacts...
Serverless: Uploading service .zip file to S3 (2.41 MB)...
Serverless: Updating Stack...
Serverless: Checking Stack update progress...
CloudFormation - UPDATE_IN_PROGRESS - AWS::CloudFormation::Stack - stresser-dev
CloudFormation - CREATE_IN_PROGRESS - AWS::IAM::Role - IamRoleLambdaExecution
CloudFormation - CREATE_IN_PROGRESS - AWS::Logs::LogGroup - HelloLogGroup
CloudFormation - CREATE_IN_PROGRESS - AWS::IAM::Role - IamRoleLambdaExecution
CloudFormation - CREATE_IN_PROGRESS - AWS::Logs::LogGroup - HelloLogGroup
CloudFormation - CREATE_COMPLETE - AWS::Logs::LogGroup - HelloLogGroup
CloudFormation - CREATE_COMPLETE - AWS::IAM::Role - IamRoleLambdaExecution
CloudFormation - CREATE_IN_PROGRESS - AWS::Lambda::Function - HelloLambdaFunction
CloudFormation - CREATE_IN_PROGRESS - AWS::Lambda::Function - HelloLambdaFunction
CloudFormation - CREATE_COMPLETE - AWS::Lambda::Function - HelloLambdaFunction
CloudFormation - CREATE_IN_PROGRESS - AWS::Lambda::Permission - HelloLambdaPermissionStresstestSNS
CloudFormation - CREATE_IN_PROGRESS - AWS::Lambda::Version - HelloLambdaVersionOVTh8yMAX1zsFvyQcL4UekM3pyWBiYTwE71y8M
CloudFormation - CREATE_IN_PROGRESS - AWS::Lambda::Permission - HelloLambdaPermissionStresstestSNS
CloudFormation - CREATE_IN_PROGRESS - AWS::SNS::Subscription - HelloSnsSubscriptionStresstest
CloudFormation - CREATE_IN_PROGRESS - AWS::Lambda::Version - HelloLambdaVersionOVTh8yMAX1zsFvyQcL4UekM3pyWBiYTwE71y8M
CloudFormation - CREATE_COMPLETE - AWS::Lambda::Version - HelloLambdaVersionOVTh8yMAX1zsFvyQcL4UekM3pyWBiYTwE71y8M
CloudFormation - CREATE_FAILED - AWS::SNS::Subscription - HelloSnsSubscriptionStresstest
CloudFormation - CREATE_FAILED - AWS::Lambda::Permission - HelloLambdaPermissionStresstestSNS
CloudFormation - UPDATE_ROLLBACK_IN_PROGRESS - AWS::CloudFormation::Stack - stresser-dev
CloudFormation - UPDATE_ROLLBACK_COMPLETE_CLEANUP_IN_PROGRESS - AWS::CloudFormation::Stack - stresser-dev
CloudFormation - DELETE_IN_PROGRESS - AWS::Lambda::Permission - HelloLambdaPermissionStresstestSNS
CloudFormation - DELETE_COMPLETE - AWS::SNS::Subscription - HelloSnsSubscriptionStresstest
CloudFormation - DELETE_COMPLETE - AWS::Lambda::Version - HelloLambdaVersionOVTh8yMAX1zsFvyQcL4UekM3pyWBiYTwE71y8M
CloudFormation - DELETE_COMPLETE - AWS::Lambda::Permission - HelloLambdaPermissionStresstestSNS
CloudFormation - DELETE_IN_PROGRESS - AWS::Lambda::Function - HelloLambdaFunction
CloudFormation - DELETE_COMPLETE - AWS::Lambda::Function - HelloLambdaFunction
CloudFormation - DELETE_IN_PROGRESS - AWS::Logs::LogGroup - HelloLogGroup
CloudFormation - DELETE_IN_PROGRESS - AWS::IAM::Role - IamRoleLambdaExecution
CloudFormation - DELETE_COMPLETE - AWS::Logs::LogGroup - HelloLogGroup
CloudFormation - DELETE_COMPLETE - AWS::IAM::Role - IamRoleLambdaExecution
CloudFormation - UPDATE_ROLLBACK_COMPLETE - AWS::CloudFormation::Stack - stresser-dev
```

Similar or dependent issues:
* #2183

## Additional Data

```
  Your Environment Information -----------------------------
     OS:                 darwin
     Node Version:       7.10.0
     Serverless Version: 1.14.0
```