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
Add unit tests for verifying extensions list

This commit adds unit tests for verify extensions functionality in
verify_tempest_config.py. It covers the use with all the currently
supported services: nova, cinder, nova_v3, neutron, and swift.

Change-Id: Idbc25ec95f1d2697dbf1994e81b83938b2e85a9f
Revert "add server personality files test"

This reverts commit 6a6f647a45b7a82246ca333f2b002aaad5855db9.

This commit added a server personality test, that in no way actually
tested server personalities. It actually just tested creating
servers. Which we do other places. So this test just adds run time
without any additional coverage. It also has substantially less
debug for the case when servers don't create effectively, so it
makes for a new place where races can happen but aren't debugged.

Change-Id: I15889b52147051e2614090e700528b363b87c498
Merge "Revert "add server personality files test""
Merge "Add unit tests for verifying extensions list"
Merge "Add a response header validation"
Merge branch 'master' of https://github.com/openstack/tempest