Set IPv4 safe address range in devstack unittests

The devstack unittests source stackrc which will fail if there is an IP
conflict with cloud networking and the default safe ip address range in
devstack. Avoid this issue by setting the value we set in d-g runs.

Change-Id: I66eb221f2ae4bb39e8e21d0e644df4ec4682dc92

jenkins/jobs/devstack.yaml

@@ -11,6 +11,9 @@
           #!/bin/bash -xe
           # This installs packages from "general" that might be
           # required for unit tests
+          # Note that we have toset the safe IP addr range to avoid
+          # conflicts with cloud networks that result in job failures.
+          echo "IPV4_ADDRS_SAFE_TO_USE=10.1.0.0/20" >> localrc
           ./tools/install_prereqs.sh
       - revoke-sudo
       - shell: |