Unhardcode image user for snat tests

Related-Prod: https://mirantis.jira.com/browse/PRODX-56051
Change-Id: I982bbab7100d7709c39a3625be00957603bad7b5
diff --git a/ironic_tempest_plugin/tests/scenario/baremetal_manager.py b/ironic_tempest_plugin/tests/scenario/baremetal_manager.py
index d08c8ce..e8fc064 100644
--- a/ironic_tempest_plugin/tests/scenario/baremetal_manager.py
+++ b/ironic_tempest_plugin/tests/scenario/baremetal_manager.py
@@ -217,7 +217,7 @@
                                                  private_key=jump_host_key)
             # TODO(mkarpin): add ability to pass proxy client to
             # get_remote_client
-            remote = ssh.Client(source_ip, "cirros",
+            remote = ssh.Client(source_ip, CONF.validation.image_ssh_user,
                                 ssh_key_type="ecdsa",
                                 pkey=private_key,
                                 proxy_client=jump_client.ssh_client)