Ensure we export correct label placement value for clustered bar charts

This is similar to 7b8073906adca8dae24c04a23708a3f3d582218f (Ensure we
export correct labal placement value for percent/stacked charts.,
2014-08-07), in case a clustered bar chart has a top placement ("t" in
OOXML), then MSO complains as well.

Change-Id: Iffd991127784e0e732f6ae55de956a328a3a53e5