enable pfcwd for backplane ports (sonic-net#3759)

Currently, in Cisco 8800 chassis, PFCWD is only enabled for front end ports, not on backplane. As we have PFC enabled for backplane ports, need to enable pfcwd there too.

What I did.
Enable PFCWD for backplane ports.

How I did it
Include backplane ports into the port list to be enabled for pfcwd

How to verify it
manually copied the file to device, and run pfcwd start_default.

Author: sdszhang

pfcwd/main.py

@@ -105,6 +105,17 @@ def get_server_facing_ports(db):
     return server_facing_ports
 
 
+def get_bp_ports(db):
+    """    Get all the backplane ports.    """
+    candidates = db.get_table('PORT')
+    bp_ports = []
+    for port in candidates:
+        if candidates[port].get('admin_status') == 'up' \
+           and candidates[port].get('role') == 'Int':
+            bp_ports.append(port)
+    return bp_ports
+
+
 class PfcwdCli(object):
     def __init__(
         self, db=None, namespace=None, display=constants.DISPLAY_ALL
@@ -365,9 +376,10 @@ def start_default(self):
         )
 
         # Get active ports from Config DB
-        active_ports = natsorted(
-            list(self.config_db.get_table('DEVICE_NEIGHBOR').keys())
-        )
+        external_ports = list(self.config_db.get_table('DEVICE_NEIGHBOR').keys())
+        bp_ports = get_bp_ports(self.config_db)
+
+        active_ports = natsorted(list(set(external_ports + bp_ports)))
 
         if not enable or enable.lower() != "enable":
             return