Epic: Support Docker's new networking model

[bfirsh]

Docker has introduced a new top-level concept of a "network", accompanied by new CLI and API functionality. Compose should support this – both out of the box, and allowing the user to manually configure complex topologies.

• Experimental support for networks - Networking #2191
• Specify networks in the Compose file - Proposal: Specify networks in the Compose file #2478
• Move network support out of experimental status and make it the default (or only) means of inter-service communication
  • Use service name and short name alises simple names in /etc/hosts when using the experimental networking feature #2312
  • Support setting the default network name Support a custom network name for default network #2470

[aanand]

Updated with TODO.

[dnephin]

I think "Support setting the default network name #2470" is supported by creating default and making it use external, but I'm not sure if we have a test for it to confirm.

[aanand]

@dnephin Done one in #2696