contrail-topology failed on no api-server

Discovery was yet to suppy the api server list during boot up, on
a multinode setup, causing topology to raise an unhandeled exception

Change-Id: I76fb7683fdcccccd58a8850cb1d317b027d91db1
Closes-Bug: 1549559

src/analytics/contrail-topology/contrail_topology/controller.py

@@ -22,7 +22,7 @@ def __init__(self, config):
         self.uve = LinkUve(self._config)
         self.sleep_time()
         self._keep_running = True
-        self._vnc = self._config.vnc_api()
+        self._vnc = None
 
     def stop(self):
         self._keep_running = False
@@ -106,7 +106,10 @@ def _chk_lnk(self, pre, index):
 
     def bms_links(self, prouter, ifm):
         if not self._vnc:
-            self._vnc = self._config.vnc_api()
+            try:
+                self._vnc = self._config.vnc_api()
+            except:
+                print 'Proceeding without any api-server'
         if self._vnc:
             try:
                 for li in self._vnc.logical_interfaces_list()[