Rename success ansible variable to zuul_success

Scope our 'success' variable from previous runs as zuul_success.

Change-Id: I96af116b9a830e5d67a9b4fd7fa7451620ad3b16
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/zuul/executor/server.py b/zuul/executor/server.py
index 03fcb4a..a9b73ce 100644
--- a/zuul/executor/server.py
+++ b/zuul/executor/server.py
@@ -1634,7 +1634,7 @@
             cmd.extend(['-e', '@' + playbook.secrets])
 
         if success is not None:
-            cmd.extend(['-e', 'success=%s' % str(bool(success))])
+            cmd.extend(['-e', 'zuul_success=%s' % str(bool(success))])
 
         if phase:
             cmd.extend(['-e', 'zuul_execution_phase=%s' % phase])