set compute min_microversion

The min microversion that Watcher needed is 2.56.[1]

[1]:https://github.com/openstack/watcher/blob/master/watcher/common/clients.py

Change-Id: If21b0df50e2f2cc2884f531cb40084e95a688026

.zuul.yaml

@@ -150,6 +150,7 @@
           $TEMPEST_CONFIG:
             compute:
               min_compute_nodes: 2
+              min_microversion: 2.56
             compute-feature-enabled:
               live_migration: true
               block_migration_for_live_migration: true