This change:
* gives us the flexibility of managing node from pull-request to pull-request. It'll be a huge lift to folks that want to test out node upgrades like @bjacobel or @andy-armstrong
* Sets us up for a port to Xenial, which will be accomplished in part by using nodeenv for managing the node environment. See a companion PR on the configuration repo for edxapp: edx/configuration#3444

This pull request does not:
* Attempt to solve any stability issues in builds related to node.
* Make node installs any faster
* Create any caching infrastructure to speed up node installs. They will be approximately as fast as they were before. Downloading the node bits build-to-build adds about 3 seconds, which is worth it for the added flexibility.