Bugfixes and enhancements to allow auditing of zabbix config from chef

Each individual commit has its own comments, but overall these enable a site to use the zabbix cookbook to audit and compare the hosts/items/triggers/templates in the zabbix database against the chef system database, so hosts removed from chef can be removed from monitoring, and monitoring items/triggers that have been added by chef can be removed if chef no longer specifies them (i.e. a host role changed, or trigger rules were changed).  (Doing that requires marking items/triggers with a site-specific string in the name when adding them, and then fetching all items/triggers with that name to compare against what the site cookbook's now say should exist.)

Also added a userparam resource type to create zabbix-agent config files in the agent's include_dir.  Each userparam resource will create a unique file with a single UserParam configuration entry.  This allows sites to configure custom monitoring items in a per-application scalable way.