[stable/train] Allow resizing of instances on same host

This allows resizing of instances on the same host and would
not depend on ssh to the other node in multinode grenade job.

For ussuri and above we'll use the zuulv3 jobs which sets it
by default.

Change-Id: I68c4ff34266ee1ae0a68df36b1fa1025b23b712a
Task: 40166

playbooks/devstack/grenade/run.yaml

@@ -31,6 +31,7 @@
           [[local|localrc]]
           enable_plugin heat https://opendev.org/openstack/heat
           TEMPEST_PLUGINS+=' ../heat-tempest-plugin'
+          NOVA_ALLOW_MOVE_TO_SAME_HOST=True
           EOF
         executable: /bin/bash
         chdir: '{{ ansible_user_dir }}/workspace'