Alternate upstream platforms via --upstream

This change, [initiated](https://github.com/ConradIrwin/kensa/commit/410afb762a770b56de195183bf74c1db40fdb2d2) by @ConradIrwin, adds a `--upstream <platform>` option to kensa for testing the local provisioning API and pushing app manifests to alternate platforms.

Various platforms use a Heroku / kensa compatible provisioning API to make it easier for third party add-on providers to integrate:

* https://broadstack.com/docs/provider-api
* https://www.cloudcontrol.com/add-on-provider-program
* http://support.appharbor.com/kb/add-ons/building-an-appharbor-add-on