Allow Varnish caching to be optional for local env

[weeirishman]

Varnishes caching can get in the way of remote API calls and services like BrowserSync when developing locally. It would be great to have the option to disable/enable it during local provisioning.

[jasoncomes]

Second this, adding this to my Provision.yml - { role: varnish, when: stage != 'local' } was a huge life saver. I'm not sure if there is a better way to doing this but varnishes caching issues for local development has always been a major pain. My votes for default to off for local dev. :)

[bostondevin]

+1