Previous implementation hardcoded "mkdir -p" which is fine for Unix, but
fails on Windows. This change draws on the example in the chef-solo
provisioner on how to detect the OS in use and use an appropriate mkdir
command.

In addition to updating the mkdir command, the actual executeCommand
needs to be OS specific, since Windows doesn't have sudo and Unix
doesn't require 'SET' when trying to change the value of a variable.

Since the Facter vars on Windows are set with 'SET <varname>=<value>', a
'&&' is needed between the SET commands and the actual Puppet
invocation.