skip stamp test until race in nova can be debugged

This test is causing a lot of gate resets, and as far as I can
tell they are really nova races in volume attachments. However
we should skip the test for now until these can be sorted out.

Related to bug #1205344

Change-Id: I983779841ac3d3f79442e96b37a0cc8da85ae55b
Merge "skip stamp test until race in nova can be debugged"