Put sphinx-build pdf logs in a separate file

Previously we would get very large console logs for the sphinx-build pdf
generation. hboxes being over and under full, problems that are likely
difficult for projects to fix and have minimal impact on the final
product. Unforatuntely, having giant console logs makes tools
like logstash and elasticsearch very unhappy. Address this by logging
this output separately.

Change-Id: I11e1bca2826287ac415683909645f4860822edff

roles/build-pdf-docs/tasks/main.yaml

@@ -8,4 +8,16 @@
     name: tox
   vars:
     tox_envlist: "{{ tox_pdf_envlist }}"
+    # We run tox against the envlist without running tests so that we can
+    # control how the tox output is handled. PDF generations tends to be
+    # incredibly verbose and we don't want those logs going to the console.
+    tox_extra_args: "-vv --notest"
   when: pdfdocs_env is success
+
+- name: Run tox with output redirected to a file
+  args:
+    chdir: "{{ zuul.project.src_dir }}"
+  shell: |
+    echo "Running tox -e {{ tox_pdf_envlist }} -vv > {{ ansible_user_dir }}/zuul-output/logs/sphinx-build-pdf.log 2>&1"
+    echo "Output is redirected to a file as it tends to be quite verbose."
+    tox -e {{ tox_pdf_envlist }} -vv > {{ ansible_user_dir }}/zuul-output/logs/sphinx-build-pdf.log 2>&1