This removes the hardcoded path to yarn.lock and allows a user to define
it manually.
Change-Id: Ib29c7d442fd57e583674ba86bdc0d5edfac844c9
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
Some javascript projects use yarn for dependencies instead of raw npm.
If a yarn.lock file is detected in the zuul_work_dir, install yarn and
use yarn to install dependencies.
Needed-By: I5864e1b2f9952fbc35b08ebdef348c4917564c37
Change-Id: I9ae7dd3b33b2b607c1c5a17632e2aeb12e339d15