Rename zero_demo Chef environment to zero-demo

From the other environment file, the convention seems to be dashes
instead of underscores in Chef names.  This patches matches the
object name with the filename for the zero-demo environment.

Change-Id: I3ecf2b07f1ac6d54943fed7b2fbf85a049fe3092
Merge "Rename zero_demo Chef environment to zero-demo"