Address vm.args tuning in Riak CS "Configuring Riak" docs

We should update the "Configuring Riak" section of Riak CS docs for 1.2.0 and 1.2.1 to include the vm.args tunings found in the Performance & Capacity Settings section of the 1.3.x docs http://docs.basho.com/riakcs/latest/cookbooks/configuration/Configuring-Riak/.