Use dhcpcd client in the tempest slow jobs

Cirros switched from udhcpc to dhcpcd in version 0.6.0.
Support for that new client is added in the Tempest in the patch [1]
and now we can use it in our slow jobs to make it green again.

[1] https://review.opendev.org/c/openstack/tempest/+/871270

Depends-On: https://review.opendev.org/c/openstack/tempest/+/871271

Closes-bug: #2003063
Change-Id: I0405a1157b785e1b628f3112a9707146cf9020f5

zuul.d/tempest-multinode.yaml

@@ -289,6 +289,10 @@
           $NEUTRON_L3_CONF:
             agent:
               debug_iptables_rules: True
+        test-config:
+          $TEMPEST_CONFIG:
+            scenario:
+              dhcp_client: dhcpcd
     group-vars:
       subnode:
         devstack_localrc:
@@ -366,6 +370,8 @@
           $TEMPEST_CONFIG:
             neutron_plugin_options:
               is_igmp_snooping_enabled: True
+            scenario:
+              dhcp_client: dhcpcd
         post-config:
           "/$NEUTRON_CORE_PLUGIN_CONF":
             ovn: