[fast reboot] allow test to replace fast-reboot script on the DUT before rebooting (#975)

- fast-reboot script is an adapted version from 201811 branch. The change is around syncd
  stop: in 201803 branch, if it is Broadcom platform, request syncd to perform cold shutdown.
- Mellanox 201803 branch has a vlan FDB issue causing all vlan IO to flood. Add a knob
  allow_vlan_flooding to ignore this symptom and continue with fast-reboot.

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

Author: yxieca

ansible/roles/test/files/ptftests/advanced-reboot.py

@@ -144,6 +144,7 @@ def __init__(self):
         self.check_param('dut_stabilize_secs', 30, required=False)
         self.check_param('preboot_files', None, required = False)
         self.check_param('preboot_oper', None, required = False)
+        self.check_param('allow_vlan_flooding', False, required = False)
         if not self.test_params['preboot_oper'] or self.test_params['preboot_oper'] == 'None':
             self.test_params['preboot_oper'] = None
 
@@ -191,6 +192,8 @@ def __init__(self):
         # second is the fast send_in_background
         self.dataplane_io_lock   = threading.Lock()
 
+        self.allow_vlan_flooding = bool(self.test_params['allow_vlan_flooding'])
+
         return
 
     def read_json(self, name):
@@ -1139,6 +1142,8 @@ def wait_dut_to_warm_up(self):
             if not self.asic_state.is_flooding() and elapsed > dut_stabilize_secs:
                 break
             if elapsed > warm_up_timeout_secs:
+                if self.allow_vlan_flooding:
+                    break
                 raise Exception("Data plane didn't stop flooding within warm up timeout")
             time.sleep(1)