Add Support of DHCP VSS Type 0 where VPN-ID is ASCII

Enhence support of DHCP VSS (Virtual Subnet Selection) to include
VSS type 0 where VSS info is a NVT (Network Virtual Terminal)
ASCII VPN ID where the ASCII string MUST NOT be terminated with a
zero byte. Existing code already support VSS type 1, where VSS
information is a RFC 2685 VPN-ID of 7 bytes with 3 bytes OUI
and 4 bytes VPN index, and VSS type 255 indicating global VPN.

Change-Id: I54edbc447c89a2aacd1cc9fc72bd5ba386037608
Signed-off-by: John Lo <loj@cisco.com>
diff --git a/test/vpp_papi_provider.py b/test/vpp_papi_provider.py
index 66126c5..3dd3482 100644
--- a/test/vpp_papi_provider.py
+++ b/test/vpp_papi_provider.py
@@ -2021,18 +2021,22 @@
 
     def dhcp_proxy_set_vss(self,
                            table_id,
-                           fib_id,
-                           oui,
+                           vss_type=255,
+                           vpn_ascii_id="",
+                           oui=0,
+                           vpn_index=0,
                            is_add=1,
                            is_ip6=0):
         return self.api(
             self.papi.dhcp_proxy_set_vss,
             {
                 'tbl_id': table_id,
-                'fib_id': fib_id,
-                'is_ipv6': is_ip6,
-                'is_add': is_add,
+                'vss_type': vss_type,
+                'vpn_ascii_id': vpn_ascii_id,
                 'oui': oui,
+                'vpn_index': vpn_index,
+                'is_add': is_add,
+                'is_ipv6': is_ip6,
             })
 
     def dhcp_client(self,