Wait for influxdb data in influxdb failover tests

Influxdb may not be ready right after service is started
for a few moments. Wait for 10 seconds for data availability.

Change-Id: I4dbe60e6271fe8415831d263646902b1ffd9ae3c
diff --git a/tcp_tests/managers/sl_manager.py b/tcp_tests/managers/sl_manager.py
index d6354b1..f65f524 100644
--- a/tcp_tests/managers/sl_manager.py
+++ b/tcp_tests/managers/sl_manager.py
@@ -255,7 +255,10 @@
             assert res['exit_code'] == 0, (
                 'Unexpected exit code for command {0}, '
                 'current result {1}'.format(cmd, res))
-            return res['stdout'][0].rstrip()
+            if res['stdout']:
+                return res['stdout'][0].rstrip()
+            else:
+                return ''
 
     def start_service(self, node_sub_name, service_name):
         target_node_name = [node_name for node_name