Devserver: delete execute_log file for provision.

Previously, execute_log for provision is preserved in devserver for possible
future investigating. However, experience shows that they're barely checked.

This CL deletes the provision execute_log after it's transferred back to
shard/drone.

BUG=chromium:654953
TEST=Run repair in local autotest with local devserver, to check whether the
file is transferred back and also deleted in /tmp/.

Change-Id: I62c6b1371eba5ca9b11c716ec1fcab111ce93efa
Reviewed-on: https://chromium-review.googlesource.com/398423
Commit-Ready: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/devserver.py b/devserver.py
index fbdbf4c..ea0d70b 100755
--- a/devserver.py
+++ b/devserver.py
@@ -960,6 +960,7 @@
         os.killpg(int(pid), signal.SIGKILL)
 
       cros_update_progress.DelTrackStatusFile(host_name, pid)
+      cros_update_progress.DelExecuteLogFile(host_name, pid)
 
     return 'True'
 
@@ -983,9 +984,10 @@
 
     host_name = kwargs['host_name']
     pid = kwargs['pid']
-    log_file = cros_update_progress.GetExecuteLogFile(host_name, pid)
-    with open(log_file, 'r') as f:
-      return f.read()
+    au_log = cros_update_progress.ReadExecuteLogFile(host_name, pid)
+    cros_update_progress.DelExecuteLogFile(host_name, pid)
+    return au_log
+
 
   @cherrypy.expose
   def locate_file(self, **kwargs):