Add a yarn role

Projects using yarn for dependencies can just run yarn commands instead
of npm commands. Add a yarn role that allows a project to do that.

Change-Id: I48a722ca55c88c6330114da3a2c035b1f84f92e1

roles/yarn/README.rst

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+Run yarn command in a source directory. Assumes the appropriate version
+of yarn has been installed.
+
+**Role Variables**
+
+.. zuul:rolevar:: yarn_command
+
+   Command to run. If it's a standard lifecycle command, it will be run as
+   ``yarn {{ yarn_command }}``. Otherwise it will be run as
+   ``yarn run {{ yarn_command }}``.
+
+.. zuul:rolevar:: zuul_work_dir
+   :default: {{ zuul.project.src_dir }}
+
+   Directory to run yarn in.

roles/yarn/defaults/main.yaml

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+zuul_work_dir: "{{ zuul.project.src_dir }}"

roles/yarn/tasks/main.yaml

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+- name: Require yarn_command variable
+  fail:
+    msg: yarn_command is required for this role
+  when: yarn_command is not defined
+
+- name: Run yarn lifecycle command
+  when: yarn_command in yarn_lifecycle_phases
+  command: "yarn {{ yarn_command }} --verbose"
+  # Need to set DISPLAY to the value that will be set when the virtual
+  # framebuffer is set up for doing browser tests.
+  environment:
+    DISPLAY: ':99'
+  args:
+    chdir: "{{ zuul_work_dir }}"
+
+- name: Run yarn custom command
+  when: yarn_command not in yarn_lifecycle_phases
+  command: "yarn run {{ yarn_command }} --verbose"
+  # Need to set DISPLAY to the value that will be set when the virtual
+  # framebuffer is set up for doing browser tests.
+  environment:
+    DISPLAY: ':99'
+  args:
+    chdir: "{{ zuul_work_dir }}"

roles/yarn/vars/main.yaml

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+yarn_lifecycle_phases:
+  - install
+  - pack
+  - publish
+  - restart
+  - start
+  - stop
+  - test
+  - version