Docker network mode selection

Enables selection of the 'NetworkMode' option present in Docker

Granted this is nearly identical to of https://github.com/ansible/ansible/pull/7408 except after some testing the 0.3.2 seems important. 

I am also not sure that not having a default actually selects mode bridge for the container, so I've added that. This is the default behaviour. This might be a problem in docker-py but I don't have tim to dig in further.

This would close https://github.com/ansible/ansible/issues/7324

Cheers,

Pauly