Opendev runs a caching mirror for npm packages. Use it when running
yarn install during tox tests. Due to the way yarn.lock files work at
the moment we need to patch the lockfile on the fly to inject the
mirror urls [1]. This can be handlel inside the pip.sh where we patch
the lockfile, run yarn and reset it afterwards.
[1] https://github.com/yarnpkg/yarn/issues/2566
Change-Id: Id1f2daac8327f3110e6beeb2d6391baccfd3ce34