Switch hdp to cdh in the default list of Sahara plugins

hdp is no longer enabled by default in Sahara, as of
https://review.openstack.org/284922 . It makes sense to remove it
by default in Tempest, to avoid failures when using default
configurations. Let's switch it to cdh, which is supported in
previous versions.

Change-Id: I7ab7fa89584c43104e5c3b297266e4641f6592a3
diff --git a/tempest/config.py b/tempest/config.py
index c3c6eda..ea151ae 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -962,7 +962,7 @@
 
 DataProcessingFeaturesGroup = [
     cfg.ListOpt('plugins',
-                default=["vanilla", "hdp"],
+                default=["vanilla", "cdh"],
                 deprecated_group="data_processing-feature-enabled",
                 help="List of enabled data processing plugins")
 ]