Support fabric's task roles

[AndrewIngram]

Fabric has a concept of roles for tasks, where you can define which groups of hosts a task should be executed on, eg 'web', 'worker', 'db', 'cache'. It would be very useful if there was a way to hook these up with the gonzo cloud configurations.

http://docs.fabfile.org/en/1.4.0/api/core/decorators.html#fabric.decorators.roles