Allow to retrieve releasenotes requirements from a dedicated place

These changes allow to store reno reqs in a dedicated place.

Indeed some projects just want to build release notes without other
documentation.

The releasenotes playbook will use the ensure-sphinx role so I think it
make sense to allow this specific use case (no doc dir available).

Change-Id: I1d244bb47fb6b0b35df130f34007ff51047cd2f1

roles/ensure-sphinx/README.rst

@@ -1,7 +1,8 @@
 Ensure sphinx is installed
 
 Installs sphinx. Also installs any dependencies needed in the first of
-doc/requirements.txt and test-requirements.txt to be found.
+doc/requirements.txt, releasenotes/requirements.txt and
+test-requirements.txt to be found.
 
 All pip installs are done with a provided constraints file, if given.
 

roles/ensure-sphinx/tasks/main.yaml

@@ -26,14 +26,14 @@
     name: find-constraints
 
 # We're not using with_first_found because the files are remote, not local.
-# We want to use doc/requirements.txt if it exists or fallback to
-# test-requirements.txt.
+# We want to use doc/requirements.txt or releasenotes/requirements.txt
+# if it exists else we want to fallback to test-requirements.txt.
 - name: Get requirements files
   shell:
     executable: /bin/bash
     chdir: "{{ zuul_work_dir }}"
     cmd: |
-      for f in doc/requirements.txt test-requirements.txt ; do
+      for f in doc/requirements.txt releasenotes/requirements.txt test-requirements.txt ; do
           if [ -f $f ] ; then
               echo $f
               break