Require explicit args for prepare_api_wrapper

This eliminates another duplication of default settings.
API keys are stored in data/users.yaml.
API version and host are stored in resource.robot

 from openprocurement_client.client import Client
 
 
-def prepare_api_wrapper(key='',
-                        host_url='https://api-sandbox.openprocurement.org',
-                        api_version='0.8'):
+def prepare_api_wrapper(key, host_url, api_version):
     return Client(key, host_url, api_version)