Set default OVS_SYSCONFDIR value depending on OVS_PREFIX

When OVN is built from source, the value of OVS_PREFIX is set to
"/usr/local". All other paths referring to OVS should be prefixed
with this value.

Closes-Bug: #1920634
Related-Bug: #1918656

Change-Id: I9a45a5379d1c47cdf67b9c6d3d0409a88501e61e
diff --git a/lib/neutron_plugins/ovn_agent b/lib/neutron_plugins/ovn_agent
index 97c20fc..2f6d1ab 100644
--- a/lib/neutron_plugins/ovn_agent
+++ b/lib/neutron_plugins/ovn_agent
@@ -111,7 +111,7 @@
 OVS_SHAREDIR=$OVS_PREFIX/share/openvswitch
 OVS_SCRIPTDIR=$OVS_SHAREDIR/scripts
 OVS_DATADIR=$DATA_DIR/ovs
-OVS_SYSCONFDIR=${OVS_SYSCONFDIR:-/etc/openvswitch}
+OVS_SYSCONFDIR=${OVS_SYSCONFDIR:-$OVS_PREFIX/etc/openvswitch}
 
 OVN_DATADIR=$DATA_DIR/ovn
 OVN_SHAREDIR=$OVS_PREFIX/share/ovn