New: auto-detect which network interface to bridge VirtualBox on

In a mixed-OS environment (e.g. a mix of Ubuntu and OSX machines), the :bridge setting in Vagrantfiles has always been a pain. There's no one setting that works equally well on multiple platforms. Until now, we've dealt with it by showing the user when Vagrant prompts for a network interface.  Thanks to the recent 'silent-subprocesses' patch, that's no longer a good idea.

Turns out that we can ask Virtualbox for a list of interfaces that are bridgeable. This PR does exactly that, and assumes that we want to bridge onto the first interface we find with an IPv4 address. (We will be tackling IPv6 support separately, and very soon). We set the environment variable VAGRANT_BRIDGE_ADAPTER to the interface that we choose, and you can use this variable inside your Vagrantfiles.