allow dashes in label keys

[networkandcode]

sls deploy was throwing a configuration error when dashes are used in keys.

$ serverless deploy
Running "serverless" from node_modules

Warning: Invalid configuration encountered
  at 'provider.labels': unrecognized property 'created-by'

Learn more about configuration validation here: http://slss.io/configuration-validation

I think dashes can be included according to the google documentation. https://cloud.google.com/deployment-manager/docs/creating-managing-labels