add the function log group id as a dependency to the subscription filter

[flemay]

There is an issue with the serverless-log-forwarding plugin when a new lambda is added to an existing serverless application. The error looks like An error occurred while provisioning your stack: SubscriptionFilterhelloWorld2 - The specified log group does not exist..

So this PR is about adding the function log group id as a dependency to ensure the LogGroup resource exists prior creating the subscription filter resource.

[flemay]

I've just looked at #7. It might be best to ignore this PR and add the dependency after #7 is merged because the code will all be in place to do something like:

makeSubscriptionFilter(arn, functionName, filterPattern) {
    const functionObject = this.serverless.service.getFunction(functionName);
    const logGroupName = this.provider.naming.getLogGroupName(functionObject.name);
    const functionLogGroupId = this.provider.naming.getLogGroupLogicalId(functionName);
    const filterLogicalId = `SubscriptionFilter${this.provider.naming.getNormalizedFunctionName(functionName)}`;
    const filter = {};
    filter[filterLogicalId] = {
      Type: 'AWS::Logs::SubscriptionFilter',
      Properties: {
        DestinationArn: arn,
        FilterPattern: filterPattern,
        LogGroupName: logGroupName,
      },
      DependsOn: [
        'LogForwardingLambdaPermission',
        functionLogGroupId,
      ],
    };
    return filter;
  }

[flemay]

I will submit another PR with better testing.