Simplify parse_image_id.

Change-Id: I7915e7a9f252da84fa76cec282c546444afa7c59
diff --git a/tempest/common/utils/data_utils.py b/tempest/common/utils/data_utils.py
index 15afd0a..c5d5c7e 100644
--- a/tempest/common/utils/data_utils.py
+++ b/tempest/common/utils/data_utils.py
@@ -58,9 +58,7 @@
 
 def parse_image_id(image_ref):
     """Return the image id from a given image ref"""
-    temp = image_ref.rsplit('/')
-    #Return the last item, which is the image id
-    return temp[len(temp) - 1]
+    return  image_ref.rsplit('/')[-1]
 
 
 def arbitrary_string(size=4, base_text=None):