Change method _sort_key_for to static

This method does not reference to any attribute of this class, so it's better
to change it to classstatic.

Change-Id: I3ea0ca83cb29ceae9afb20da0c9817a9b044c084
Signed-off-by: Lin Yang <lin.a.yang@intel.com>
Merge "Change method _sort_key_for to static"