Clean up duplicate network

[bboreham]

Fixes #1047.

It's not entirely satisfactory, as it relies on the cleanup working in a particular way. However client.create_network() does not return the ID cleanly in the previous call (it returns something like {"u'Warning': u'Network with name dockerpy845377 (id : 3d8a876ff38eaa6df14d37a40922046b1274fb685c5cf25abfe71c94531e1c41) already exists', u'Id': u'4cf5bbce6cb471730fbd962c5391392c3708dc4bfc3b720ec44c047c38e2f5ef'"})

[GordonTheTurtle]

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

$ git clone -b "remove-network" git@github.com:bboreham/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

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

[GordonTheTurtle]

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

$ git clone -b "remove-network" git@github.com:bboreham/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

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

[bboreham]

@GordonTheTurtle it's signed now.

[shin-]

Thanks, LGTM!

[bboreham]

I'm not sure what your bot didn't like about my signature, but evidently re-signing it made it happy.

Have also re-based against master.

[shin-]

Ah, sorry I lost track of this, but I actually included that fix in #1178 . Thanks again for your help tracking this down!