Skip get_server_diagnostics test until bug 1240043 is fixed

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

Bug 1240043 has been raised to define a hypervisor-agnostic API which can be
tested and maintained

Closes bug: 1239209

Change-Id: I3e643d8e7a63b7dc42a912fab09ffc4222d463db
Merge "Skip get_server_diagnostics test until bug 1240043 is fixed"