jenkins_worker: Add pre-populated .nmprc file to jenkins home dir.

@jzoldak @hkim823 @clytwynec 

This is the companion piece to https://github.com/edx/edx-platform/pull/4828. The npm profile should be on the worker already, and when the get registry call is made, the set registry call (which would result in errno 50) is unnecessary and therefore avoided.


Verified on vagrant (ubuntu precise machine).

```
---> pavelib.prereqs.install_node_prereqs
test `npm config get registry` = "http://registry.npmjs.org/" || (echo setting registry; npm config set registry http://registry.npmjs.org/)
npm install
coffee-script@1.6.3 node_modules/coffee-script
```