Change set -> default in attribute files to prevent node definition pollution.

A set attribute permanently changes a node's json definition. When a cookbook is depended upon the attributes file is included.

This means that when a node is given the 'nagios::client' recipe the attribute files for nagios, php, and apache are all included.

This causes attributes set at the "normal" level for everything from 'nagios::server' to 'mysql::server' to be set in a 'nagios::client's node definition. This pollution reduces readability and causes confusion.

In the attached commit I've converted all "set" attributes to "default" this will allow the recipes to function identically without adjusting a node's definition unnecessarily.