Clean Train::Platform#clean_name.
Clean Train::Platform#clean_name.

Bang methods can return nil, and cause problems. Use functional forms
instead. Once that's in place, it really doesn't need the begin/end
and extra naming to still be clear.

Signed-off-by: Ryan Davis <f3417dae5443a381b6f3c0abee902614d44fe9d3@chef.io>

chef/train,chef/train