Fix wrong exceptions import from test_metadata

SSHExecCommandFailed should be imported from tempest.lib.exceptions, not
from neutron_tempest_plugin.exceptions

Change-Id: I74e3e69d9e7ca866840f7a9f1e1990d88a138da0
diff --git a/neutron_tempest_plugin/scenario/test_metadata.py b/neutron_tempest_plugin/scenario/test_metadata.py
index 05f0f04..af6bd09 100644
--- a/neutron_tempest_plugin/scenario/test_metadata.py
+++ b/neutron_tempest_plugin/scenario/test_metadata.py
@@ -19,11 +19,11 @@
 from tempest.common import utils
 from tempest.lib.common.utils import data_utils
 from tempest.lib import decorators
+from tempest.lib import exceptions
 import testtools
 
 from neutron_tempest_plugin.common import ssh
 from neutron_tempest_plugin import config
-from neutron_tempest_plugin import exceptions
 from neutron_tempest_plugin.scenario import base
 
 LOG = logging.getLogger(__name__)