First stab at making the provisioning setup easier to configure.

This modifies the installer script to accept an environment variable `PROVISIONING_INTERFACE` to use, instead of guessing the interface. If no interface specified, defaults to guessing.

This also allows the installer to use the same interface for provisioning as the gateway interface.

Lastly, tweaks the default dhcp range to 50-100, since this interferes with /25 nets. Ideally there should be logic to derive the proper range.