COOK-2826 - Allow Nagios cookbook to configure location of SSL files

- Create attributes for SSL Certificate File and
  SSL Certificate Key locations. Default them to the self
  generated nagios-server.pem file.
- Use consistent naming of variables in the apache2 and
  nginx configuration files. This passes variables to the
  templates and renderes the SSL configuration if @https is true.
  The @https variable is determined by what the enable_ssl variable
  is set to.

[COOK-2826](http://tickets.opscode.com/browse/COOK-2826)