Fix eventlet.tpool import

Currently nova.utils.tpool_execute() only works by chance. And as the
bug report shows there are env where it fails.

The nova.utils.tpool_execute() call tries to uses eventlet.tpool.execute
but the tpool module is not imported by the utils module only eventlet.
In devstack it works by chance as the wsgi init actually imports
eventlet.tpool indirectly via:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/stack/nova/nova/api/openstack/compute/__init__.py", line 21, in <module>
    from nova.api.openstack.compute.routes import APIRouterV21 # noqa
  File "/opt/stack/nova/nova/api/openstack/compute/routes.py", line 20, in <module>
    from nova.api.openstack.compute import admin_actions
  File "/opt/stack/nova/nova/api/openstack/compute/admin_actions.py", line 17, in <module>
    from nova.api.openstack import common
  File "/opt/stack/nova/nova/api/openstack/common.py", line 27, in <module>
    from nova.compute import task_states
  File "/opt/stack/nova/nova/compute/task_states.py", line 26, in <module>
    from nova.objects import fields
  File "/opt/stack/nova/nova/objects/fields.py", line 24, in <module>
    from nova.network import model as network_model
  File "/opt/stack/nova/nova/network/model.py", line 23, in <module>
    from nova import utils
  File "/opt/stack/nova/nova/utils.py", line 39, in <module>
    from oslo_concurrency import processutils
  File "/usr/local/lib/python3.8/dist-packages/oslo_concurrency/processutils.py", line 57, in <module>
    from eventlet import tpool

This was broken since I8dbc579e0037969aab4f2bb500fccfbde4190726. This
patch adds the correct import statement.

Change-Id: Ic46345ceeb445164aea6ae9b35c457c6150765f6
Closes-Bug: #1915400
(cherry picked from commit b2d28f8)
(cherry picked from commit a9f444a)

Author: Balazs Gibizer

nova/utils.py

@@ -29,6 +29,7 @@
 import tempfile
 
 import eventlet
+from eventlet import tpool
 from keystoneauth1 import loading as ks_loading
 import netaddr
 from openstack import connection
@@ -684,7 +685,7 @@ def context_wrapper(*args, **kwargs):
 
 def tpool_execute(func, *args, **kwargs):
     """Run func in a native thread"""
-    eventlet.tpool.execute(func, *args, **kwargs)
+    tpool.execute(func, *args, **kwargs)
 
 
 def is_none_string(val):