Convert the output of 'arp -n' from bytes to str. (#1279)

The output of 'arp -n' is bytes in python3, as a result, the search of
'ether' or other str failed. This commit address the issue.

Signed-off-by: bingwang <bingwang@microsoft.com>

scripts/nbrshow

@@ -106,7 +106,7 @@ class NbrBase(object):
         """
             Fetch Neighbor data (ARP/IPv6 Neigh) from kernel.
         """
-        p = subprocess.Popen(self.cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        p = subprocess.Popen(self.cmd, shell=True, text=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         (output, err) = p.communicate()
         rc = p.wait()