Add attributes for pickling

[AnTuanHa]

When using Python's multiprocessing module, it throws an AttributeError, complaining that the object does not have the attribute used. This adds the missing attributes and allows them to be pickled.

[AnTuanHa]

@shin- Any chance on gettting this merged? Here's a minimal working example of the error occurring:

Create a local docker-machine: docker-machine create -d virtualbox testmachine
Then run:

import docker
import multiprocessing as mp
import re
import subprocess

def foo(client):
    pass

if __name__ == '__main__':
    cmd = ['docker-machine', 'config', 'testmachine']
    p = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    stdout, _ = p.communicate()
    output = stdout.decode('utf-8')
    regex = """(--tlsverify\n)?--tlscacert="(.+)"\n--tlscert="(.+)"\n--tlskey="(.+)"\n-H=(.+)"""
    match = re.match(regex, output.strip())
    tlsverify, tlscacert, tlscert, tlskey, host = match.group(1, 2, 3, 4, 5)
    tlsverify = bool(tlsverify)

    if tlsverify:
        base_url = host.replace('tcp://', 'https://')
        tls = docker.tls.TLSConfig(client_cert=(tlscert, tlskey),
                                   ca_cert=tlscacert,
                                   verify=True)


        client = docker.DockerClient(base_url=base_url, tls=tls)

        pool = mp.Pool()
        job = pool.apply_async(foo, args=[client])
        job.wait()

The error:

Process ForkPoolWorker-1:
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 249, in _bootstrap
    self.run()
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 108, in worker
    task = get()
  File "/usr/lib/python3.6/multiprocessing/queues.py", line 345, in get
    return _ForkingPickler.loads(res)
  File "/tmp/testlocal/lib/python3.6/site-packages/requests/adapters.py", line 142, in __setstate__
    block=self._pool_block)
  File "/tmp/testlocal/lib/python3.6/site-packages/docker/transport/ssladapter.py", line 40, in init_poolmanager
    'assert_hostname': self.assert_hostname,
AttributeError: 'SSLAdapter' object has no attribute 'assert_hostname'

[shin-]

LGTM - should we do a similar thing with UnixAdapter and NpipeAdapter as well?

[AnTuanHa]

@shin- Yup, also rebased on top of latest upstream/master.

[shin-]

Great, thank you!