Ensure default timeout is used by API Client

[grahamlyons]

The from_env method on the docker module passed None as the value for the timeout keyword argument which overrode the default value in the initialiser, taken from constants module.

This sets the default in the initialiser to None and adds logic to set that, in the same way that version is handled.

This sets the default in from_env to the default defined in the constants module.

Addresses #1374

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "readtimeout_calling_container_stop" [email protected]:grahamlyons/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[grahamlyons]

Is the "janky" check configured correctly? It doesn't have a link to a Jenkins job and it hasn't completed on the last large handful of pull requests.

[grahamlyons]

I'm not sure what the failure on Jenkins relates to; can anybody assist with that?

[shin-]

Please ignore the janky test... It's an obsolete test, but we somehow haven't been able to disable the corresponding webhook.

[shin-]

My only issue with this PR is that None has a special meaning for timeouts (requests docs).

[grahamlyons]

@shin-, is that the kind of change you envisaged? Let me know if not.

[shin-]

Yeah, I think that works! Thank you!

[grahamlyons]

Awesome. Thanks, @shin-