Reduce the "minimal" set of required diagnostics

Change If631472081d16fe8c0b37575eb157a7b13d6fba4 introduced new restrictions
on the diagnostics API which are not valid with XenAPI and causes it to fail
tempest tests.

I haven't found documentation which describes the minimum set of diagnostics
so therefore I assume we can safely reduce this test to ensure a truly minimal
set

Closes bug: 1239209

Change-Id: I3e643d8e7a63b7dc42a912fab09ffc4222d463db