Mark test_minimum_basic_instance_hard_reboot_after_vol_snap_deletion as slow test

We are seeing more timeout in tests (and so does sometimes it end up to
job timeout) in various jobs. A few examples are

- miltinode job
  - https://zuul.openstack.org/build/084729c5a1fe46e091a17a50441e99d5/log/job-output.txt

- stable/yoga job
  - https://zuul.opendev.org/t/openstack/build/9b79f54d3f6c4e05ae6619fdac7ad95a

Two tests in doubt and taking more time is
test_minimum_basic_instance_hard_reboot_after_vol_snap_deletion
- taking 258.358054s
  - https://zuul.openstack.org/build/084729c5a1fe46e091a17a50441e99d5/log/job-output.txt#34173
- taking 187.411266s
  - https://zuul.opendev.org/t/openstack/build/9b79f54d3f6c4e05ae6619fdac7ad95a/log/job-output.txt#25738

test_minimum_basic_scenario 
- taking 309.109043s
  - https://zuul.opendev.org/t/openstack/build/d068cb494d234fe7b79dc5ae6fd6ae69/log/job-output.txt#24052
  
marking these test as slot test and monitor if there is another slow
test we can find.

Related-Bug: #2004780
Change-Id: I0aff3507b3bf3498ab0ecd548bb57cdcd97ec11a

tempest/scenario/test_minimum_basic.py

@@ -86,6 +86,7 @@ class TestMinimumBasicScenario(manager.ScenarioTest):
                    '%s' % (secgroup['id'], server['id']))
             raise exceptions.TimeoutException(msg)
 
+    @decorators.attr(type='slow')
     @decorators.idempotent_id('bdbb5441-9204-419d-a225-b4fdbfb1a1a8')
     @utils.services('compute', 'volume', 'image', 'network')
     def test_minimum_basic_scenario(self):
@@ -159,6 +160,7 @@ class TestMinimumBasicScenario(manager.ScenarioTest):
                 self.servers_client, server, floating_ip,
                 wait_for_disassociate=True)
 
+    @decorators.attr(type='slow')
     @decorators.idempotent_id('a8fd48ec-1d01-4895-b932-02321661ec1e')
     @testtools.skipUnless(CONF.volume_feature_enabled.snapshot,
                           "Cinder volume snapshots are disabled")