Allow default encoding to be defined in config file

This patch fixes the bug
https://bugs.launchpad.net/opencontrail/+bug/1362156

Change-Id: I6637ff3fb5432c5c6809134a3380786b0ff1d588
Closes-bug: #1362156
(cherry picked from commit 8e766700dbd76460aa3b1f6ebd1c4406e7da3532)