Memory leak

[zhusichuang]

api = core_v1_api.CoreV1Api()
resp = stream(api.connect_get_namespaced_pod_exec, name, 'default',
              command=exec_command,
              stderr=True, stdin=False,
              stdout=True, tty=False)
api = None

The python GC can't collect the variable 'api',and api.api_client.pool.close()won't be called.This will make my process have many threads until it crashes.

[roycaihw]

Dup of #411