Rewrite tarball URLs to always go through kappa/internal host

[totherik]

Currently, all tarball urls from public repo include the hostname registry.npmjs.org, thus once the initial json payload is downloaded via kappa, subsequent tarball requests are made of the public repo. This change will funnel all traffic through kappa, but more importantly through the private npm host/dns, such that caching can be added to the pipeline without messing with registry.npmjs.org via dns.

So instead of:

client --> kappa --> private couchdb -\
     \---------------------------------\---> public repo

you could support

client --> kappa --> private couchdb --> private cache --> public repo