Multiprovider support

This patchset implements multiprovider support for AutoNetwork by adding a new `PoolManager` class. The `PoolManager` creates `Pool` objects for each Vagrant provider. Each `Pool` manages a `/24` range of IP addresses.

Also, the identifier used to associate Vagrant machines with specific IP addresses has changed. Instead of using the VirtualBox UUID, a Hash consisting of the `Vagrantfile` path and VM name is used.

**Caveats**

Currently, this is a backwards-incompatible change. You may need to re-generate `~/.vagrant.d/auto_networ/pool.yaml`. Consider making a backup copy.