Custom SSH Credentials

I can't rely on an ssh agent to provide a private key. Sometimes I need to provide a password, sometimes I need to manually load the key file. But the docker-py code only connects with this in SSHHTTPAdapter:

self.ssh_client.connect(
    parsed.hostname, parsed.port, parsed.username,
)

There is no flexibility here. How am I supposed to connect with a custom key or password? I thought I might hack my way in by reassigning APIClient._custom_adapter to my own subclass of SSHHTTPAdapter, but then I realized the APIClient.__init__ is a huge mess that does way too much. That method would always raise an exception so I would also have to totally reimplement that method in a subclass. This is too much maintenance overhead for my deployment script that I would like to keep as simple as possible.

It should be exposed in SSHHTTPAdapter. I might even recommend doing both of these:

1. Add optional key and password parameters to SSHHTTPAdapter so you don't have to write your own subclass of it just to use custom credentials
2. Add optional ssh_client parameter to SSHHTTPAdapter to be used instead of instantiating one in __init__.

It should also be exposed somehow in APIClient.__init__, for example:

1. Expose password and private key with...
   a. Explicit parameters (a bit messy considering this is not an SSH specific class)
   b. General purpose configuration object (dict or better yet custom configuration class)
2. Enable programmer to provide their own SSHClient to APIClient. This is not ideal for the same reason as 1a.
3. Enable programmer to provide their own instance of a BaseHTTPAdapter to be used as self._custom_adapter. I think this is the best solution but there are some open questions. Like what's the best way to handle all the usages of base_url when it is no longer required to establish a connection.

I'm happy to take the lead on this and submit a PR, but I would like to get some feedback first.

Start by reading SSHHTTPAdapter and APIClient.init, focusing on how SSH credentials, the SSH client, and the custom adapter are currently constructed. Review the open design questions before making changes; done means an agreed API supports custom SSH credentials or clients without requiring a maintained subclass.