fixing variable name, and adding @ to clear up master warnings

Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Failed to parse template mongodb/debian_mongos-init.conf.erb:
  Filepath: /usr/lib/ruby/vendor_ruby/puppet/parser/templatewrapper.rb
  Line: 81
  Detail: Could not find value for 'mongod_instance' at /etc/puppet/environments/development/modules/mongodb/templates/debian_mongos-init.conf.erb:24
 at /etc/puppet/environments/development/modules/mongodb/manifests/mongos.pp:22 on node mongoprototype1.dallas07.softlayer.georiot.com
Warning: Not using cache on failed catalog
Error: Could not retrieve catalog; skipping run