Remove trivial cases of unused variables (1)

Kill some of the variables marked as unused by flakes8. This should
allow to enable F841 check in the future.
Only trivial cases with no function calls and obviously pure functions
(like datetime.now(), or len()) are cleaned up here.

Part 1, split to reduce conflicts.

Change-Id: I82854349574ec4bcb9336aba626eefdaed81a8c8

nova/scheduler/filters/trusted_filter.py

@@ -153,7 +153,7 @@ class AttestationService(object):
                 return httplib.OK, res
             return status_code, None
 
-        except (socket.error, IOError) as e:
+        except (socket.error, IOError):
             return IOError, None
 
     def _request(self, cmd, subcmd, hosts):

nova/service.py

@@ -286,7 +286,7 @@ class Service(service.Service):
         """Perform basic config checks before starting processing."""
         # Make sure the tempdir exists and is writable
         try:
-            with utils.tempdir() as tmpdir:
+            with utils.tempdir():
                 pass
         except Exception as e:
             LOG.error(_('Temporary directory is invalid: %s'), e)