Add a wrapper installation script that will install npm and yarn into the
current virtualenv. If yarn is installed globally, the script is a
no-op.
If yarn is not installed globally, whenever tox thinks it needs to
create or re-create a virtualenv, install nodeenv then use it to
install yarn.
This removes the use of the zuul-tox-py35 job because it should now just work
properly with the normal job. It does not remove the job itself because
it's still used in tox-py35-on-zuul.
Change-Id: If360a3f0c6b3d74498f8aa063d8b1ae87daff101
Monty Taylor
fadd15a001