Add a flavor test for getting a specified key value API

Test for GET v2/.{tenant_id}./flavors/.{flavor_id}./os-extra_specs/
.{key_id} that gets the value of the specified key for this flavor.

Change-Id: Id9941604aff9c583b9f2d394ba2299aad7dafec8
Simplify xml/json client selection

The simpler/shorter xml/json switch code,
 makes easer the client related changes.

Change-Id: Ia5ca3cbeeb2577e6d8d8a46d2dbe1f1506375fae
Merge "Simplify xml/json client selection"
Merge "Add a flavor test for getting a specified key value API"