[COOK-3702] Allow multiple pagerduty contacts/keys.

Change-Id: Ic8a7f5abf93de97ddfe914b38d53aae18b26f04f
Organize the client attributes in the readme and add missing attributes
Add additional chef spec tests for client installs
Work around the lack of _default hostgroup when using multi_environment_monitoring
Added the ability to exclude specific hostnames from being monitored by use of the ['nagios']['server']['exclude_hosts'] array