Support for using the ServiceNet ip address for SSH connections

Adds an optional config setting `:servicenet`, which defaults to `false`.

When `true`, the driver will use the first private address of the
node instead of the first public address.  The first private ip
address should always be the ServiceNet interface (10.184.0.0/13
or 10.208.0.0/12).

The ServiceNet interface may be preferable when running kitchen
on another Rackspace cloud server, like a Jenkins build server,
or for customers with VPN access to ServiceNet.

Using ServiceNet avoids bandwidth charges and may avoid changes
to the PublicNet/RackConnect firewall rules to allow
SSH connections to the kitchen test VMs from the public Internet.