Allow to specify the list of sahara enabled plugins

The list of plugins can be tuned by cloud administrator, so make
sure that the list of checked plugins is not hardcoded.
The default list of plugins matches the one currently enabled.

This commit introduces also a new section for data_processing enabled
features in the tempest configuration.

Change-Id: Ie29aeef2df605cfe840e053bc57ff980e9332636
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 02609ae..3ade411 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -467,6 +467,16 @@
 #endpoint_type = publicURL
 
 
+[data_processing-feature-enabled]
+
+#
+# From tempest.config
+#
+
+# List of enabled data processing plugins (list value)
+#plugins = vanilla,hdp
+
+
 [database]
 
 #