Move api_version to a class value

Volume v2 service clients pass api_version in __init__() now, but
Iddd8306723c1ff33105f513c1993a0497a949c29 will move the passed
api_version to class values for avoiding a redundant __init__()
definition.
For doing this, we need to move api_version to class value to avoid
initializing it in RestClient __init__().

Change-Id: Ic86739dde83dcac8f68e53599967de53694f692f

tempest_lib/common/rest_client.py

@@ -39,6 +39,9 @@ class RestClient(object):
 
     TYPE = "json"
 
+    # The version of the API this client implements
+    api_version = None
+
     LOG = logging.getLogger(__name__)
 
     def __init__(self, auth_provider, service, region,
@@ -54,8 +57,6 @@ class RestClient(object):
         self.build_timeout = build_timeout
         self.trace_requests = trace_requests
 
-        # The version of the API this client implements
-        self.api_version = None
         self._skip_path = False
         self.general_header_lc = set(('cache-control', 'connection',
                                       'date', 'pragma', 'trailer',