Add new snapshot compute feature flag

Nova image creation / instance snapshotting is not supported across compute
drivers.   This adds a new compute feature flag and skips tests that attempt
to create snapshots.

Change-Id: I814157d434e3d5e13dfae9b9ee7d5135199c4e97
diff --git a/tempest/scenario/test_stamp_pattern.py b/tempest/scenario/test_stamp_pattern.py
index be27024..4783273 100644
--- a/tempest/scenario/test_stamp_pattern.py
+++ b/tempest/scenario/test_stamp_pattern.py
@@ -16,6 +16,7 @@
 import time
 
 from cinderclient import exceptions as cinder_exceptions
+import testtools
 
 from tempest.common.utils import data_utils
 from tempest import config
@@ -150,6 +151,8 @@
         self.assertEqual(self.timestamp, got_timestamp)
 
     @tempest.test.skip_because(bug="1205344")
+    @testtools.skipUnless(CONF.compute_feature_enabled.snapshot,
+                          'Snapshotting is not available.')
     @tempest.test.services('compute', 'network', 'volume', 'image')
     def test_stamp_pattern(self):
         # prepare for booting a instance