Add a response header validation

Nova API sometimes returns important information in a response header,
and Tempest needs to check it for keeping the backward compatibility.
This patch adds the check for "create a server snapshot" API as the
sample.

Partially implements blueprint nova-api-attribute-test

Change-Id: I2441147ee050e7b818906aaa754dcec4480d8c18
Merge "Add a response header validation"