Create yarn_lock_file_path variable for install-yarn

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>

roles/install-yarn/README.rst

@@ -6,3 +6,8 @@ Install yarn from yarnpkg repos
    :default: {{ zuul.project.src_dir }}
 
    The directory to work in.
+
+.. zuul:rolevar:: yarn_lock_file_path
+   :default: {{ zuul_work_dir }}/yarn.lock
+
+   Path to yarn.lock file used by a project.

roles/install-yarn/defaults/main.yaml

@@ -1 +1,2 @@
 zuul_work_dir: "{{ zuul.project.src_dir }}"
+yarn_lock_file_path: "{{ zuul_work_dir }}/yarn.lock"

roles/install-yarn/tasks/main.yaml

@@ -1,6 +1,6 @@
 - name: Check for yarn.lock file
   stat:
-    path: "{{ zuul_work_dir }}/yarn.lock"
+    path: "{{ yarn_lock_file_path }}"
     get_checksum: false
     get_mime: false
     get_md5: false