Fix issue that show dhcp_relay vlan shows only the first dhcp helper

      Signed-off-by: Ping Mao <pmao@linkedin.com>

      RB=3132724
      G=lnos-reviewers
      R=pchaudha,pmao,samaity,zxu,stexu,pgohite
      A=zxu,pgohite

show/main.py

@@ -1170,11 +1170,11 @@ def vlan(vid, verbose):
     body = []
     if vid is not None:
         vlan = 'Vlan'+str(vid)
-        if vlan in vlan_dict.keys():
-                body.append([vlan, vlan_dict[vlan]['dhcp_servers'][0].encode("utf-8")])
+        if vlan in vlan_dict:
+                body.append([vlan, '\n'.join(vlan_dict[vlan]['dhcp_servers'] if 'dhcp_servers' in vlan_dict[vlan] else [])])
     else:
-        for vlan in vlan_dict.keys():
-                body.append([vlan, vlan_dict[vlan]['dhcp_servers'][0].encode("utf-8")])
+        for vlan in vlan_dict:
+                body.append([vlan, '\n'.join(vlan_dict[vlan]['dhcp_servers'] if 'dhcp_servers' in vlan_dict[vlan] else [])])
 
     click.echo(tabulate(body, header))