add tests for show and update of Flavor Extra Spec API extension

this adds positive and negative  tests for show and update actions
of FlavorExtra Spec API extension.
this checks the extends response attribute that added by all flavor
extension API.
this also move some translation from xml to json into the xml flavor
client, so tests can use xml client and json client in the same way.

Change-Id: I0cf47640a61da8a062ea14b49554911cab8b3be7
Merge "add tests for show and update of Flavor Extra Spec API extension"