Ansible launcher: Append to console log, rather than truncating

Jobs end up having lots of builders, each should append to the log.
We remove the log before the job starts so we don't need to worry
about truncating it here on re-used nodes.

Change-Id: I287968b600de0eae48c1bdcf2f91c4383578731c
diff --git a/zuul/ansible/library/zuul_runner.py b/zuul/ansible/library/zuul_runner.py
index 68318e7..cba640f 100644
--- a/zuul/ansible/library/zuul_runner.py
+++ b/zuul/ansible/library/zuul_runner.py
@@ -22,7 +22,7 @@
 
 class Console(object):
     def __enter__(self):
-        self.logfile = open('/tmp/console.txt', 'w+', 0)
+        self.logfile = open('/tmp/console.txt', 'a', 0)
         return self
 
     def __exit__(self, etype, value, tb):