Skip over updating requirement files that are missing

This returns backwards compatibility of configuration versions
before 09807f8e.

(cherry picked from commit 25cb491d6465b1f743a46c198443a8332ada1dff)

playbooks/roles/edxapp/defaults/main.yml

@@ -548,6 +548,7 @@ edxapp_aa_command: "{% if EDXAPP_SANDBOX_ENFORCE %}aa-enforce{% else %}aa-compla
 # all edxapp requirements files
 edxapp_requirements_with_github_urls:
   - "{{ pre_requirements_file }}"
+  - "{{ post_requirements_file }}"
   - "{{ base_requirements_file }}"
   - "{{ paver_requirements_file }}"
   - "{{ github_requirements_file }}"

playbooks/roles/edxapp/tasks/deploy.yml

@@ -60,12 +60,19 @@
     GIT_SSH: "{{ edxapp_git_ssh }}"
   register: edxapp_theme_checkout
 
+- name: Stat each requirements file to ensure it exists
+  stat: path="{{ item }}"
+  with_items: "{{ edxapp_requirements_with_github_urls }}"
+  register: requirement_file_stats
+
 # Substitute github mirror in all requirements files
 # This is run on every single deploy
 - name: Updating requirement files for git mirror
   command: |
-    /bin/sed -i -e 's/github\.com/{{ COMMON_GIT_MIRROR }}/g' {{ " ".join(edxapp_requirements_with_github_urls) }}
+    /bin/sed -i -e 's/github\.com/{{ COMMON_GIT_MIRROR }}/g' {{ item.item }}
   sudo_user: "{{ edxapp_user }}"
+  when: item.stat.exists
+  with_items: "{{ requirement_file_stats.results }}"
 
 # Ruby plays that need to be run after platform updates.
 - name: gem install bundler