Replace the usage of 'admin_manager' with 'os_admin'

In tempest, alias 'admin_manager' has been moved to 'os_admin'in
version Pike, and it will be removed in version Queens[1].

[1]I5f7164f7a7ec5d4380ca22885000caa0183a0bf7

Change-Id: Ic29cb510a558ceee832fbfae7853106decffbb41
Closes-bug: 1697588
diff --git a/ironic_tempest_plugin/tests/manager.py b/ironic_tempest_plugin/tests/manager.py
index 343ac9f..1a451ec 100644
--- a/ironic_tempest_plugin/tests/manager.py
+++ b/ironic_tempest_plugin/tests/manager.py
@@ -95,7 +95,7 @@
         self.baremetal_client.set_node_provision_state(self, uuid, state)
 
     def hypervisor_stats(self):
-        return (self.admin_manager.hypervisor_client.
+        return (self.os_admin.hypervisor_client.
                 show_hypervisor_statistics())
 
     def server_show(self, uuid):