os_vif: Add support for configuring os_vif_ovs plugin

This change add an os-vif lib that declares two new variables
OS_VIF_OVS_OVSDB_INTERFACE and OS_VIF_OVS_ISOLATE_VIF

The former is introduced to workaround bug #1929446 which cause the nova
and neutron agents to periodically block waiting for ovs to respond.

OS_VIF_OVS_ISOLATE_VIF is added to address bug #1734320 when using
ml2/ovs vif isolation should always be used to prevent cross tenant
traffic during a live migration.  This makes devstack more closely
mirror reality by enabling it when ml2/ovs is used and disabling it
otherwise.

Related-Bug: #1734320
Related-Bug: #1929446
Related-Bug: #1912310
Change-Id: I88254c6e22b52585506ee4907c1c03b8d4f2dac7
diff --git a/stack.sh b/stack.sh
index 6858ab8..44f1c8f 100755
--- a/stack.sh
+++ b/stack.sh
@@ -597,6 +597,7 @@
 source $TOP_DIR/lib/dstat
 source $TOP_DIR/lib/tcpdump
 source $TOP_DIR/lib/etcd3
+source $TOP_DIR/lib/os-vif
 
 # Extras Source
 # --------------
@@ -1159,6 +1160,11 @@
     sudo sysctl -w net.ipv4.ip_forward=1
 fi
 
+# os-vif
+# ------
+if is_service_enabled nova neutron; then
+    configure_os_vif
+fi
 
 # Storage Service
 # ---------------