Skip volume snapshot tests if feature is not enabled

Not all cinder volume backends support volume snapshot. For instance,
the default NFS backend doesn't. This patch adds a new entry to
CONF.volume_feature_enabled to add the possibility to skip volume
snapshot tests.

Change-Id: I40ce17924bbe4eb6f1c941df974bfa832ec2a999
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 761a077..87f08f4 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -954,6 +954,9 @@
 # Runs Cinder volumes backup test (boolean value)
 #backup=true
 
+# Runs Cinder volume snapshot test (boolean value)
+#snapshot=true
+
 # A list of enabled volume extensions with a special entry all
 # which indicates every extension is enabled (list value)
 #api_extensions=all