NAT added FIB entries have a preference lower than API/CLI

Change-Id: Ia99490180683e8649784f7d9d18c509c3ca78438
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/fib/fib_entry.h b/src/vnet/fib/fib_entry.h
index 92333e9..ddaf914 100644
--- a/src/vnet/fib/fib_entry.h
+++ b/src/vnet/fib/fib_entry.h
@@ -73,6 +73,10 @@
      */
     FIB_SOURCE_CLI,
     /**
+     * A low (below routing) priority source a plugin can use
+     */
+    FIB_SOURCE_PLUGIN_LOW,
+    /**
      * LISP
      */
     FIB_SOURCE_LISP,
@@ -169,6 +173,7 @@
     [FIB_SOURCE_URPF_EXEMPT] = "urpf-exempt",	        \
     [FIB_SOURCE_DEFAULT_ROUTE] = "default-route",	\
     [FIB_SOURCE_PLUGIN_HI] = "plugin-hi",               \
+    [FIB_SOURCE_PLUGIN_LOW] = "plugin-low",             \
     [FIB_SOURCE_INTERPOSE] = "interpose",               \
 }