[COOK-3440] Nagios Cookbook does not permit additional pagerduty-like integrations.

Refactor the pagerduty recipe to add a contact to a mash, instead of hard-coding the support into the template.

Reported ticket: http://tickets.opscode.com/browse/COOK-3440

For an example on why this is useful, see this cookbook which adds nagios -> hipchat integration: https://github.com/zippykid/chef-nagios-hipchat