Sometimes projects are multi-language. Specifically, sometimes they need
to run both javascript and python toolchains. Support this by making a
tox-nodejs-npm job which installs both toolchains, then runs the npm
command followed by the tox command.
Most people won't need/want this, but it should make at least some folks
happy.
Change-Id: Iaedd680a548c6e8159693b7a79cd8676e805aa1f
91d1126097