change .data vrs .meta file metadata filtering in obj.diskfile

Add DATAFILE_SYSTEM_META to diskfile.py which is a set of
system-set metadata keys that cannot be changed with a POST.

Fixes: bug #1214607

Change-Id: I4bdfc1e4813a1d27fe726ba83481c6e7194aab7f
Merge "change .data vrs .meta file metadata filtering in obj.diskfile"