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/cros_update_progress.py b/cros_update_progress.py
index 17a60e0..9c69352 100644
--- a/cros_update_progress.py
+++ b/cros_update_progress.py
@@ -85,11 +85,21 @@
   return TRACK_LOG_FILE_PATH % (host_name, pid)
 
 
+def ReadExecuteLogFile(host_name, pid):
+  """Return the content of execute log file."""
+  return osutils.ReadFile(GetExecuteLogFile(host_name, pid))
+
+
 def DelTrackStatusFile(host_name, pid):
   """Delete the track status log."""
   osutils.SafeUnlink(GetTrackStatusFile(host_name, pid))
 
 
+def DelExecuteLogFile(host_name, pid):
+  """Delete the track status log."""
+  osutils.SafeUnlink(GetExecuteLogFile(host_name, pid))
+
+
 class AUProgress(object):
   """Used for tracking the CrOS auto-update progress."""