Puppet: serve 'puppet/files' as custom mountpoint

This patch configures Puppet's file server to serve files under 'puppet/files'
as 'puppet:///files/...'. This provides a means for role classes (which are not
part of modules, and hence cannot rely on Puppet's automatic serving of module
file directories) to provision files.

Change-Id: I9022e881c65a8a0c5aa01babe2f09c8576f52dfc