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.

Also, help with an echo.

Some Jenkins builds are erroring when attempting to set up the
npm environment with the error:

`npm ERR! cb() never called!`

Although it could point to a couple of different things, it is more classically
a sign of a dirty npm environment cache. Cleaning the cache at the start
of any given Jenkins job (which are sourced through this file) should
alleviate the problem.

Sass compilation is no longer dependent on Ruby, so we are
removing its dependencies.

The jenkins-common script is the better place for this particular setup, since that
script is doing similar setup steps to prepare the environment for running
various tests/subsequent scripts. This also will resolve downstream processes,
such as our jenkins AMI builder (for jenkins-worker images).