Install devstack "general" prereqs for unit tests

Use the existing "install_prereqs.sh" script to install the packages
from the "general" package list, which might be required to run
unit-tests.  Do this before we revoke sudo so that we can still run
the unit-tests unprivileged.

Change-Id: Ib47d802a31a0f4c2a49daa7e6698e37c70a2365a
Related-Change: I77c8c2019fb8b8174cdfaed3e56ebf728f0732b7

jenkins/jobs/devstack.yaml

@@ -7,6 +7,11 @@
     builders:
       - gerrit-git-prep
       - install-distro-packages
+      - shell: |
+          #!/bin/bash -xe
+          # This installs packages from "general" that might be
+          # required for unit tests
+          ./tools/install_prereqs.sh
       - revoke-sudo
       - shell: |
           #!/bin/bash -xe