Fix libffi on periodic jobs

We applied this to gate-*-python26 a while back, but missed applying
it to periodic jobs. Also, this yum fail is making me thisty.

Change-Id: Ibd835cb6565e497406a099ae660fd5719a830173

modules/openstack_project/files/jenkins_job_builder/config/python-bitrot-jobs.yaml

@@ -7,6 +7,12 @@
       - timestamps
 
     builders:
+      - shell: |
+          #!/bin/bash -xe
+          if ! rpm -ql libffi-devel >/dev/null ; then
+              sudo yum install -y libffi-devel
+          fi
+          rpm -ql libffi-devel
       - revoke-sudo
       - branch-git-prep:
           branch: '{branch}'