Disable DHCP server on tested devices in DHCP client integration tests

When testing the 'dnsmasq' device as a DHCP client, the DHCP client
flag is removed from the interface (because the device is by default a
DHCP server), resulting in validation failure.

The plugin introduced in this commit removes the 'dhcp.server' flag
from nodes in the 'dhcp_clients' group to ensure the DHCP clients are
never also DHCP servers, causing the DHCP feature check to fail on
dnsmasq DHCP clients.

Author: ipspace

tests/integration/dhcp/01-ipv4-client.yml

@@ -1,4 +1,18 @@
 ---
+message: |
+  Use this topology to test the DHCPv4 client on your device. Use 'netlab
+  validate' to check whether your devices requested DHCP leases from the DHCP
+  server.
+
+  If your client suppors configurable 'router option' requests to generate
+  a default route, check that C1 has a default route but C2 does not.
+
+plugin: [ no_dhcp_server ]
+
+groups:
+  dhcp_clients:
+    members: [ c1, c2 ]
+
 nodes:
   c1:
   c2:
@@ -31,11 +45,3 @@ validate:
     valid:
       eos: len(vrfs.default.ipv4ActiveLeases) == 2
       dnsmasq: stdout.rstrip() == '2'
-
-message: |
-  Use this topology to test the DHCPv4 client on your device. Use 'netlab
-  validate' to check whether your devices requested DHCP leases from the DHCP
-  server.
-
-  If your client suppors configurable 'router option' requests to generate
-  a default route, check that C1 has a default route but C2 does not.

tests/integration/dhcp/02-ipv6-client.yml

@@ -1,3 +1,15 @@
+---
+message: |
+  Use this topology to test the DHCPv6 client on your device. Use 'netlab
+  validate' to check whether your devices requested DHCPv6 leases from the
+  DHCPv6 server.
+
+plugin: [ no_dhcp_server ]
+
+groups:
+  dhcp_clients:
+    members: [ c1, c2 ]
+
 addressing.lan:
   ipv4: false
   ipv6: 2001:db8:cafe::/48
@@ -33,8 +45,3 @@ validate:
     valid:
       eos: vrfs.default.ipv6ActiveLeases
       dnsmasq: stdout.rstrip() == '2'
-
-message: |
-  Use this topology to test the DHCPv6 client on your device. Use 'netlab
-  validate' to check whether your devices requested DHCPv6 leases from the
-  DHCPv6 server.

tests/integration/dhcp/no_dhcp_server.py

@@ -0,0 +1,13 @@
+"""
+A plugin that removes dhcp.server flag from DHCP clients -- needed to make
+dnsmasq fail the DHCP client test at "create the lab" phase
+"""
+
+from box import Box
+
+
+def pre_link_transform(topology: Box) -> None:
+  clients = topology.get('groups.dhcp_clients.members',[])
+  for c_node in clients:
+    c_data = topology.nodes[c_node]
+    c_data.pop('dhcp.server',True)