allow overwriting search order for passwd, shadow and group in nsswitch....

Previously nsswitch.conf was a static file that clobbered everything in place.  This change allows some customization of the file.  The intent to allow this customization seemed already to be in place, as nsswitch.conf.erb was already a template; it, just, was not in use.

These are the customizations I needed.  It would probably be a good idea to add the rest of the options as variables.

I can't see this file being modified on the system too much; I can't see that using Augeas makes sense.