make async pool thread count configurable

and default to 1 for putting requests in the background
without triggering concurrency.

modules/swagger-codegen/src/main/resources/python/api_client.mustache

@@ -40,6 +40,8 @@ class ApiClient(object):
         the API.
     :param cookie: a cookie to include in the header when making calls
         to the API
+    :param pool_threads: The number of threads to use for async requests
+        to the API. More threads means more concurrent API requests.
     """
 
     PRIMITIVE_TYPES = (float, bool, bytes, six.text_type) + six.integer_types
@@ -56,10 +58,11 @@ class ApiClient(object):
     _pool = None
 
     def __init__(self, configuration=None, header_name=None, header_value=None,
-                 cookie=None):
+                 cookie=None, pool_threads=1):
         if configuration is None:
             configuration = Configuration()
         self.configuration = configuration
+        self.pool_threads = pool_threads
 
         self.rest_client = rest.RESTClientObject(configuration)
         self.default_headers = {}
@@ -82,7 +85,7 @@ class ApiClient(object):
         avoids instantiating unused threadpool for blocking clients.
         """
         if self._pool is None:
-            self._pool = ThreadPool()
+            self._pool = ThreadPool(self.pool_threads)
         return self._pool
 
     @property