Construct a list of members from a configurable node variable, and utilize it for pagerduty.
Use a mash, and set the contact in server.rb

Using a Mash allows for simpler merging and manipulation, while
preventing duplicate entries. Additionally allows roles or environments
to disable an entry.

Set the contact in the server.rb attribute, by @jtimberman's suggestion
in order to prevent attribute-sprawl.
Add an example of the additional_contacts within the README
Fix an issue where pagerduty_key is always true, always adding pagerduty to the contacts.
Merge pull request #185 from zippykid/additional-contact-members

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