Add new DEVNODES option for devrel builds

With Riak version 1.3 we can now `make devrel` and specify the number of nodes desired, like so:

`make devrel DEVNODES=N`

We should update the relevant docs to note this handy feature:

* [Building a Development Environment](http://docs.basho.com/riak/latest/tutorials/fast-track/Building-a-Development-Environment/)
* [Basic Cluster Setup](http://docs.basho.com/riak/latest/cookbooks/Basic-Cluster-Setup/)