Fixing capitalization of SubElement in wrappers.py

The locks function was broken due to a mis-capitalization
It was XML.Subelement and I changed it to XML.SubElement
The locks functionality now works correctly

Change-Id: Icebcad27515eebce9609262e0f9f3d5c87acb19f
Reviewed-on: https://review.openstack.org/18687
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins