[CHEF-3690] Restore run_chef_client private method relied upon by Windows...

http://tickets.opscode.com/browse/CHEF-3690

This fix restores a private method run_chef_client that was removed from the WindowsService class, even though the class still attempts to call this method in its service_main method. Besides meaning that the class itself is fundamentally broken, this causes the functionality of WindowsService to break.

The commit that removed this class appears to be https://github.com/opscode/chef/commit/04bdf7152d0fd88db7281d1675846b82bac66634#diff-3

The ticket that tracks the effect on customers is http://tickets.opscode.com/browse/CHEF-3690. T

Impact is that anyone attempting to use the chef gem's ability to install itself as a Windows service can no longer do so. This fix restores the functionality by simply adding back the private method.