Change heat_subnet to heat-subnet

At this moment runtest created subnet with name 'heat_subnet'
but heat-tempest-plugin expect to have 'heat-subnet'.

Change-Id: I938d2cd0baa8100740365448dfcf5ab9776fa9df
Related-PROD: PROD-22018
diff --git a/metadata/service/tempest/neutron.yml b/metadata/service/tempest/neutron.yml
index 6d0a05f..b159012 100644
--- a/metadata/service/tempest/neutron.yml
+++ b/metadata/service/tempest/neutron.yml
@@ -10,7 +10,7 @@
               admin_state_up: True
               gateway_network: ${_param:runtest_tempest_public_net}
               interfaces:
-                - heat_subnet
+                - heat-subnet
           network:
             heat-net:
               tenant: admin
@@ -18,7 +18,7 @@
               router_external: False
               admin_state_up: True
               subnet:
-                heat_subnet:
+                heat-subnet:
                   cidr: 10.20.30.0/24
                   allocation_pools:
                     - start: 10.20.30.10