Fixes bug for PFCWD feature parameters (#838)

What I did

The feature allows setting 'detection_time', 'restoration_time' and 'pollling_interval' PFCWD to an interface. The 'pollling_interval' must be lower than 'detection_time' and 'restoration_time'.
The fix is checking if there is a lower value of 'detection_time' or 'restoration_time' than the 'pollling_interval' value entered by the user in config DB, if yes exit with error code 1.

How I did it
Checking the config DB for interfaces PFCWD values.

How to verify it
Try adding 'pollling_interval' greater than one of the values of PFCWD interfaces.

Previous command output (if the output of a command-line utility has changed)
No Output.

New command output (if the output of a command-line utility has changed)
unable to use polling_interval = #ms, value is bigger or equal to the minimum in PFCWD table

pfcwd/main.py

@@ -3,6 +3,7 @@
 import click
 import swsssdk
 import os
+import sys
 from tabulate import tabulate
 from natsort import natsorted
 
@@ -194,9 +195,25 @@ def interval(poll_interval):
     configdb.connect()
     pfcwd_info = {}
     if poll_interval is not None:
+        pfcwd_table = configdb.get_table(CONFIG_DB_PFC_WD_TABLE_NAME)
+        entry_min = 3000
+        for entry in pfcwd_table:
+            if("Ethernet" not in entry):
+                continue
+            detection_time_entry_value = int(configdb.get_entry(CONFIG_DB_PFC_WD_TABLE_NAME, entry).get('detection_time'))
+            restoration_time_entry_value = int(configdb.get_entry(CONFIG_DB_PFC_WD_TABLE_NAME, entry).get('restoration_time'))
+            if ((detection_time_entry_value != None) and (detection_time_entry_value < entry_min)):
+                entry_min = detection_time_entry_value
+                entry_min_str = "detection time"
+            if ((restoration_time_entry_value != None) and (restoration_time_entry_value < entry_min)):
+                entry_min = restoration_time_entry_value
+                entry_min_str = "restoration time"
+        if entry_min < poll_interval:
+            print >> sys.stderr, "unable to use polling interval = {}ms, value is bigger than one of the configured {} values, please choose a smaller polling_interval".format(poll_interval,entry_min_str)
+            exit(1)
+
         pfcwd_info['POLL_INTERVAL'] = poll_interval
-
-    configdb.mod_entry(CONFIG_DB_PFC_WD_TABLE_NAME, "GLOBAL", pfcwd_info)
+        configdb.mod_entry(CONFIG_DB_PFC_WD_TABLE_NAME, "GLOBAL", pfcwd_info)
 
 # Stop WD
 @cli.command()