Define NEUTRON_ENDPOINT_SERVICE_NAME variable as empty string

This new Devstack's variable is introduced by patch [1] but as it's not
in the stable branches of Devstack yet, it can't be used in Grenade jobs
with default setting.
So lets set it to the emtpy string to make it working as it was before
[1].

[1] https://review.opendev.org/c/openstack/devstack/+/849145

Change-Id: I2de65d4764f23ae85a086d4d09ed80c81777434b

.zuul.yaml

@@ -85,6 +85,10 @@
           Q_AGENT: openvswitch
           Q_ML2_TENANT_NETWORK_TYPE: vxlan
           Q_ML2_PLUGIN_MECHANISM_DRIVERS: openvswitch
+          # TODO(slaweq): It can be removed when devstack patch
+          # https://review.opendev.org/c/openstack/devstack/+/849145 will be
+          # merged in master and last stable branch
+          NEUTRON_ENDPOINT_SERVICE_NAME: ""
       grenade_localrc:
         SAVE_DIR: "{{ devstack_bases.shared }}/save"
       # from tempest playbooks