Fix a pointer to NotFound exception

Noticed an error in a run like this:

 Captured traceback-1:
 ~~~~~~~~~~~~~~~~~~~~~
     Traceback (most recent call last):
       File "tempest/api/compute/base.py", line 437, in _detach_volume
         except exceptions.NotFound:
     AttributeError: 'module' object has no attribute 'NotFound'

This exception was moved (I guess) but this spot was missed.

Closes-Bug: #1733674

Change-Id: Ia61eafe58ffc9658f70a8867c7729555a4e1d91c
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index 705814c..9ee8858 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -434,7 +434,7 @@
             # the compute API will return a 400 response.
             if volume['status'] == 'in-use':
                 self.servers_client.detach_volume(server['id'], volume['id'])
-        except exceptions.NotFound:
+        except lib_exc.NotFound:
             # Ignore 404s on detach in case the server is deleted or the volume
             # is already detached.
             pass