Skip Nova API attribute tests if an XML response

XML client of Tempest is broken and cannot get some keys from Nova API
response body. The best way is to fix it, but now XML format of Nova
API has been marked as "deprecated" and XML client will be removed
from Tempest.

In this situation, this patch makes the Nova attribute tests disable
if an XML response.

Partially implements blueprint nova-api-attribute-test

Change-Id: I1a482df79959786dd1ed7662b305acf0a8062747
Merge "Skip Nova API attribute tests if an XML response"