WIP: Cloud Service Deploy Parametrization

:construction: WORK IN PROGRESS :construction:

This is the functionality added on the support request [Cloud Service Deployment Parametrisation](http://help.appveyor.com/discussions/questions/423-cloud-service-deployment-parametrisation), but is, as yet, undocumented.

This first draft is mostly copied from the [web deploy parametrization section](http://www.appveyor.com/docs/deployment/web-deploy#web-deploy-parametrization).

I'm initially raising the PR to confirm this is something desirable, I'll flesh it out with screen shots and a better description later.