Kitchen test: use node_6.x repo for ubuntu-1604
Kitchen test: use node_6.x repo for ubuntu-1604

Ubuntu 16.04 canonical packages are currently using nodejs 4.x, whereas
the old node repo is on 0.10.x. This makes the ubuntu package take
precedence, causing `npm` binary to not be included in nodejs package.

I will also submit a PR to nodejs cookbook to handle new nodejs repos
there.

djoos-cookbooks/newrelic,djoos-cookbooks/newrelic,djoos-cookbooks/newrelic