Disable pfcwd forward action config for cisco-8000. (sonic-net#1848)

Disable pfcwd forward action config for cisco-8000.
Do not configure pfcwd if pfc is not configured on a port
This PR is in association with: sonic-net/sonic-swss#1748 where PFC-WD forward action is disabled for cisco-8000 platform.

Signed-off-by: Alpesh S Patel alpesh@cisco.com

What I did
There are two changes of interest:
- disables pfcwd forward action configuration from pfcwd command
- in the pfcwd script, adds a check to enable pfcwd configuration on a port only if it has pfc enabled. If config_db.json has an empty entry like: "pfc_enable" : "", and subsequently pfcwd is configured, swss/orchagent throws an error logs

How to verify it
- can be verified on cisco-8000 platform. It is a no-op for other platforms
# pfcwd start -a forward 400 -r 400
SKIPPED: PFC WD 'forward' action not supported on asic 'cisco-8000'

- for a single port, in /etc/sonic/config_db.json, set no pfc enable bits by setting:
"pfc_enable" :  "" for atleast 1 port.
without this change, see the logs in syslog. With this change, there will just be a log on console.

pfcwd/main.py

@@ -11,7 +11,7 @@
 from tabulate import tabulate
 from utilities_common import multi_asic as multi_asic_util
 from utilities_common import constants
-from sonic_py_common import logger
+from sonic_py_common import device_info, logger
 
 SYSLOG_IDENTIFIER = "config"
 
@@ -59,6 +59,7 @@
 
 CONFIG_DB_PFC_WD_TABLE_NAME = 'PFC_WD'
 PORT_QOS_MAP =  "PORT_QOS_MAP"
+UNSUPPORTED_FORWARD_ACTION_ASICS = ["cisco-8000"]
 
 # Main entrypoint
 @click.group()
@@ -117,6 +118,8 @@ def __init__(
         )
         self.table = []
         self.all_ports = []
+        version_info = device_info.get_sonic_version_info()
+        self.asic_type = version_info.get('asic_type')
 
     @multi_asic_util.run_on_multi_asic
     def collect_stats(self, empty, queues):
@@ -250,7 +253,7 @@ def start(self, action, restoration_time, ports, detection_time):
 
     def verify_pfc_enable_status_per_port(self, port, pfcwd_info):
         pfc_status = self.config_db.get_entry(PORT_QOS_MAP, port).get('pfc_enable')
-        if pfc_status is None:
+        if pfc_status is None or pfc_status == "":
             log.log_warning("SKIPPED: PFC is not enabled on port: {}".format(port), also_print_to_console=True)
             return
 
@@ -274,6 +277,10 @@ def start_cmd(self, action, restoration_time, ports, detection_time):
         if len(ports) == 0:
             ports = all_ports
 
+        if action == 'forward' and self.asic_type in UNSUPPORTED_FORWARD_ACTION_ASICS:
+            log.log_warning("SKIPPED: PFC WD 'forward' action not supported on asic '{}'".format(self.asic_type), also_print_to_console=True)
+            return
+
         pfcwd_info = {
             'detection_time': detection_time,
         }