This helps with some confusion caused in GH-2538, since the output says:
> Running cleanup tasks for 'shell' provisioner...
But that's actually not true. It is running the cleanup tasks iff the
provisioner defined a cleanup task. This commit changes the
provisioner_cleanup middleware to only run cleanup tasks if the subclass
defines a cleanup task.
The reason we can't just check if the provisioner `respond_to?` the
`cleanup` method is because the parent provisioner base class (which
all provisioners inherit from) defines a blank cleanup method. This is
important because it means we never risk calling an unimplemented
cleanup function, and it also helps define the public API for a
provisioner.
