Add DHCPv6 to minigraph.py and improve unit test (sonic-net#8125)

Why I did it
Allow deploying DHCPv6 servers following the implementation PR: sonic-net#7772

How I did it
Add DHCPv6 to minigraph.py on sonic-cfggen tool and improve the unit test to cover this change.

How to verify it
Try to deploy a switch with DHCPv6 servers.

Signed-off-by: Shlomi Bitton <shlomibi@nvidia.com>

src/sonic-config-engine/minigraph.py

@@ -562,6 +562,12 @@ def parse_dpg(dpg, hname):
                 vdhcpserver_list = vintfdhcpservers.split(';')
                 vlan_attributes['dhcp_servers'] = vdhcpserver_list
 
+            vintf_node = vintf.find(str(QName(ns, "Dhcpv6Relays")))
+            if vintf_node is not None and vintf_node.text is not None:
+                vintfdhcpservers = vintf_node.text
+                vdhcpserver_list = vintfdhcpservers.split(';')
+                vlan_attributes['dhcpv6_servers'] = vdhcpserver_list
+
             vlanmac = vintf.find(str(QName(ns, "MacAddress")))
             if vlanmac is not None and vlanmac.text is not None:
                 vlan_attributes['mac'] = vlanmac.text
@@ -803,6 +809,7 @@ def filter_bad_asn(table):
 def parse_meta(meta, hname):
     syslog_servers = []
     dhcp_servers = []
+    dhcpv6_servers = []
     ntp_servers = []
     tacacs_servers = []
     mgmt_routes = []
@@ -856,7 +863,7 @@ def parse_meta(meta, hname):
                     kube_data["enable"] = value
                 elif name == "KubernetesServerIp":
                     kube_data["ip"] = value
-    return syslog_servers, dhcp_servers, ntp_servers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data
+    return syslog_servers, dhcp_servers, dhcpv6_servers, ntp_servers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data
 
 
 def parse_linkmeta(meta, hname):
@@ -1170,6 +1177,7 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
     mux_cable_ports = {}
     syslog_servers = []
     dhcp_servers = []
+    dhcpv6_servers = []
     ntp_servers = []
     tacacs_servers = []
     mgmt_routes = []
@@ -1217,7 +1225,7 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
             elif child.tag == str(QName(ns, "UngDec")):
                 (u_neighbors, u_devices, _, _, _, _, _, _) = parse_png(child, hostname, None)
             elif child.tag == str(QName(ns, "MetadataDeclaration")):
-                (syslog_servers, dhcp_servers, ntp_servers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data) = parse_meta(child, hostname)
+                (syslog_servers, dhcp_servers, dhcpv6_servers, ntp_servers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data) = parse_meta(child, hostname)
             elif child.tag == str(QName(ns, "LinkMetadataDeclaration")):
                 linkmetas = parse_linkmeta(child, hostname)
             elif child.tag == str(QName(ns, "DeviceInfos")):
@@ -1567,6 +1575,7 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
         results['DEVICE_NEIGHBOR_METADATA'] = { key:devices[key] for key in devices if key in {device['name'] for device in neighbors.values()} }
     results['SYSLOG_SERVER'] = dict((item, {}) for item in syslog_servers)
     results['DHCP_SERVER'] = dict((item, {}) for item in dhcp_servers)
+    results['DHCPv6_SERVER'] = dict((item, {}) for item in dhcpv6_servers)
     results['NTP_SERVER'] = dict((item, {}) for item in ntp_servers)
     results['TACPLUS_SERVER'] = dict((item, {'priority': '1', 'tcp_port': '49'}) for item in tacacs_servers)
     results['ACL_TABLE'] = filter_acl_table_bindings(acls, neighbors, pcs, sub_role)

src/sonic-config-engine/tests/t0-sample-graph.xml

@@ -256,6 +256,7 @@
           <FhrpProtoType i:nil="true"/>
           <Type i:nil="true"/>
           <DhcpRelays>192.0.0.1;192.0.0.2</DhcpRelays>
+          <Dhcpv6Relays>fc02:2000::1;fc02:2000::2</Dhcpv6Relays>
           <VlanID>1000</VlanID>
           <Tag>1000</Tag>
           <Subnets>192.168.0.0/27</Subnets>