Set proper OVS_SYSCONFDIR for the job wher OVS is compiled

Devstack's patch [1] introduced new variable OVS_SYSCONFDIR which by
default points to the /etc/openvswitch directory. But in case of the
jobs where OVS is installed from source there is no such directory and
/usr/local/etc/openvswitch should be used instead.

[1] https://review.opendev.org/c/openstack/devstack/+/779991

Change-Id: I240d0df197e89224c2447625130f02d640116ba1
Related-Bug: #1918656
diff --git a/zuul.d/master_jobs.yaml b/zuul.d/master_jobs.yaml
index 0b0f174..d5afab6 100644
--- a/zuul.d/master_jobs.yaml
+++ b/zuul.d/master_jobs.yaml
@@ -368,6 +368,7 @@
         OVN_BUILD_FROM_SOURCE: True
         OVN_BRANCH: "v20.12.0"
         OVS_BRANCH: "branch-2.15"
+        OVS_SYSCONFDIR: "/usr/local/etc/openvswitch"
       devstack_services:
         br-ex-tcpdump: true
         br-int-flows: true