Add a job variable to control javascript link copying

The zuul multi-tenant dashboard job wants to make a few symlinks to
simulate what would happen on a real deployment with rewrite rules. The
current hard-coded behavior of copying the links causes ... havoc.

Add a job variable defaulting to the current behavior which such jobs
can flip if they need to.

Change-Id: I751524baa80353606d74b87dbaacfe7cd8b71a5e

roles/fetch-javascript-output/README.rst

@@ -12,3 +12,9 @@ Collect outputs from a javascript build
 
    Directory, relative to zuul_work_dir, in which javascript output content
    is to be found.
+
+.. zuul:rolevar:: javascript_copy_links
+   :default: true
+
+   Whether to copy the data pointed to by symlinks in the built content, or
+   to copy them as symbolic links.

roles/fetch-javascript-output/defaults/main.yaml

@@ -1,2 +1,3 @@
 zuul_work_dir: "{{ zuul.project.src_dir }}"
 javascript_content_dir: dist
+javascript_copy_links: true

roles/fetch-javascript-output/tasks/main.yaml

@@ -115,6 +115,6 @@
     src: "{{ zuul_work_dir }}/{{ javascript_content_dir }}/"
     dest: "{{ log_path }}/html/"
     mode: pull
-    copy_links: true
+    copy_links: "{{ javascript_copy_links }}"
     verify_host: true
   when: javascript_output.stat.exists