OpenStackService doesn't support passing network ID for spawned instances

OpenStack novaclient supports passing a network ID for spawned instances to attach them to the correct network in a tenant where there are multiple networks. This is however not supported by OpenStackService right now.

OpenStack has some support for designating a network and a subnet pool as default via auto-allocated network topology https://docs.openstack.org/newton/networking-guide/config-auto-allocation.html .

Most of the time we'd want to be able to fix it to a particular subnet ID, a private one for dockerDriver and a public one (probably) for openstackdriver.

The documentation on novaclient is a bit limited so reading the code is the best approach https://github.com/openstack/python-novaclient/blob/master/novaclient/v2/servers.py#L758