Doesn't validate keys in yaml

[Guneetgstar]

I met this issue with a typo error where this plugin threw a runtime error:

Cannot read property 'replace' of undefined
      at ServerlessAppsyncPlugin.getCfnName 
     .
     .

I figured out that although even my mapping templates were being validated, the 'dataSource' field was not checked if present or not as I miss spelled it as 'datasource' and also 'dataSources' as 'datasources'

I think the default behaviour should be to include a validation on the 'appSync' config. Anyways its ok if not, this might help someone else.