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.


Use the `COMMON_NPM_MIRROR` url instead of hard coding this.

https://github.com/edx/configuration/blob/4ea4e16a03a20e4cd179dce640a158e990af57d7/playbooks/roles/common/defaults/main.yml#L26