Fix forgotten import of `exceptions`.

File "tests/compute/base.py", line 107, in BaseComputeTest._get_isolated_creds
NameError: global name 'exceptions' is not defined

Change-Id: If106274c9a0c3fca41e1bea2abf264f34c2dd461
diff --git a/tempest/tests/compute/base.py b/tempest/tests/compute/base.py
index 5e6eb7d..e1b7bdd 100644
--- a/tempest/tests/compute/base.py
+++ b/tempest/tests/compute/base.py
@@ -22,6 +22,7 @@
 
 from tempest import config
 from tempest import openstack
+from tempest import exceptions
 from tempest.common.utils.data_utils import rand_name
 from tempest.services.identity.json.admin_client import AdminClient