Designate API tests for Tempest Framework

Creates a new folder 'tempest' under 'designate/contrib' as a
placeholder for Designate API tests.
This submission implements CRUD operations on 'domains', 'records'
and 'servers' of Designate Api.
Adds related JSON supporting methods in client files
'domains_client.py', 'records_client.py', and 'server_client.py'
Also implemented schema validation for all the operations under
'domains.py', 'parameter_types.py', 'records.py', 'servers.py' files.
Modified 'tempest.conf.sample', 'config.py' , and 'clients.py' to
add parameters for enabling designate service.

Change-Id: I5fcac06200ce534a9cddf1e7ed9393ae5124a681
Merge "Designate API tests for Tempest Framework"