Use TempestException from tempest.lib

Fix for error:
AttributeError: 'module' object has no attribute 'TempestException'

Change-Id: I4caf4a57226dd711575cde766076fa25d16792e2
diff --git a/tempest/api/compute/admin/test_auto_allocate_network.py b/tempest/api/compute/admin/test_auto_allocate_network.py
index 4eb3376..8e481fd 100644
--- a/tempest/api/compute/admin/test_auto_allocate_network.py
+++ b/tempest/api/compute/admin/test_auto_allocate_network.py
@@ -19,8 +19,8 @@
 from tempest.common import credentials_factory as credentials
 from tempest.common import waiters
 from tempest import config
-from tempest import exceptions
 from tempest.lib.common.utils import test_utils
+from tempest.lib import exceptions as lib_excs
 from tempest import test
 
 CONF = config.CONF
@@ -82,14 +82,14 @@
         nets = cls.networks_client.list_networks(
             **search_opts).get('networks', [])
         if nets:
-            raise exceptions.TempestException(
+            raise lib_excs.TempestException(
                 'Found tenant networks: %s' % nets)
         # (2) Retrieve shared network list.
         search_opts = {'shared': True}
         nets = cls.networks_client.list_networks(
             **search_opts).get('networks', [])
         if nets:
-            raise exceptions.TempestException(
+            raise lib_excs.TempestException(
                 'Found shared networks: %s' % nets)
 
     @classmethod