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/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 9ace4ea..247f6d1 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -474,6 +474,10 @@
 # attachment? (boolean value)
 #interface_attach=true
 
+# Does the test environment support creating snapshot images
+# of running instances? (boolean value)
+#snapshot=true
+
 
 [dashboard]