Simplify lambda datasource config

[bboure]

Basically, this will simplify lambda datasources and allow you to do

- type: AWS_LAMBDA
    name: myLambdaDatasource
    description: 'My Lambda Datasource'
    config:
      functionName: myLambda

Where myLambda is the key name of your lambda in your serverless.yml

instead of

- type: AWS_LAMBDA
    name: myLambdaDatasource
    description: 'My Lambda Datasource'
    config:
      lambdaFunctionArn: { Fn::GetAtt: [MyLambdaLambdaFunction, Arn] }

This will also work with AppSync Emulator, and will prevent you from having to do this:

- type: AWS_LAMBDA
    name: myLambdaDatasource
    description: 'My Lambda Datasource'
    config:
      functionName: myLambda
      lambdaFunctionArn: { Fn::GetAtt: [MyLambdaLambdaFunction, Arn] }

[bboure]

Any review from anyone that has a fresh/recent view on the code is welcome.