CI: fix kayobe-tox-molecule job

The kayobe-tox-molecule job is currently failing on stable/train and
earlier branches. This was caused by the 13.0 release of the 'sh'
package [1]. A 13.1 release fixed this for Python 3, but there is no fix
available for Python 2 currently.

This change works around the issue by pinning 'sh' to <13.0 in
test-requirements.txt.

[1] ansible/molecule#2676

Change-Id: I0df1dadb3b48736cdbd25112517e524533550210
(cherry picked from commit 5f83651)

test-requirements.txt

@@ -8,6 +8,8 @@ coverage>=4.0 # Apache-2.0
 doc8 # Apache-2.0
 docker<3 # Apache-2.0
 hacking>=0.12.0,<0.13 # Apache-2.0
+# sh 1.13.1 causes molecule yamllint to fail.
+sh<1.13 # MIT
 molecule<3 # MIT
 oslotest>=1.10.0 # Apache-2.0
 # NOTE(mgoddard): This is required by tools/sphinx8.