Add a toggle to disable ARA static report generation

Without this patch, it is not possible for jobs to disable the
ara report generation because it is part of the base job by default.

It might be useful (or required) for jobs to disable the static
report generation if there are issues with it or if the static report
is causing problems. This toggle will provide that ability.

Change-Id: Ia5e43db9f852d405ea987b380e36cb93c983314d

roles/emit-ara-html/README.rst

@@ -2,6 +2,11 @@ Have ARA generate html logs if ARA and ARA data are both present.
 
 **Role Variables**
 
+.. zuul:rolevar:: ara_generate_html
+
+   Whether to generate a static ARA HTML report or not.
+   Defaults to ``true``.
+
 .. zuul:rolevar:: ara_compress_html
 
    Whether to compress the ARA HTML output or not.

roles/emit-ara-html/defaults/main.yaml

@@ -1,2 +1,5 @@
+# Whether to generate a static ARA HTML report or not
+ara_generate_html: true
+
 # Whether to compress the ARA HTML output or not
 ara_compress_html: true

roles/emit-ara-html/tasks/main.yaml

@@ -12,7 +12,10 @@
 - name: Generate ARA html output
   command: "ara generate html {{ zuul.executor.log_root }}/ara"
   ignore_errors: yes
-  when: ara_command_type|succeeded and not ara_command_type|skipped
+  when:
+    - ara_command_type | succeeded
+    - not ara_command_type | skipped
+    - ara_generate_html | bool
 
 - name: Compress ARA html output
   command: gzip --recursive --best {{ zuul.executor.log_root }}/ara