Set selinux context on file after shutil.move in atomic_move()

This is intended to fix #2810.  It sets the context of the tmp_dest file
after shutil.move() operation and before os.rename().  This should
retain the selinux context of the file across moves.
Merge pull request #2817 from sfromm/issue2810

Set selinux context on file after shutil.move in atomic_move()