Allow docker0 CIDR to be specified

# NOTE: This pull request depends on #21

## Problem

Using dvm in an environment where the default network range for Docker (`172.16.42.1/24`) conflicts with the local network, containers are unable to reach any hosts on the local network.

Docker allows passing in the `--bip=""` option to set the CIDR range for the `docker0` bridge it creates. However, this alone is not enough. Since Docker will have already been started once, the `docker0` bridge will already exist with the conflicting network range, causing the `--bip` option to take no effect.

## Solution

* To allow setting the CIDR range for the `docker0` bridge, I've added the configuration directive `DOCKER0_CIDR` to `dvm.conf`.

* To fix the issue of `docker0` previously existing, the `bridge-utils` package is installed to provide `brctl` (I did check busybox first to make sure it was not already included), which is then used to remove the `docker0` bridge before starting Docker with the new arguments.