Allow to specify networks to which NICs will be connected when creating VM

This allows to specify networks to which VM will be connected, when the openstack has multiple networks.
This is passed to the attribute "networks" in create server request.
This will work fine on fog the commit https://github.com/fog/fog/commit/9938c92516a4e82d092f54f8982cf4ed6f2cc250 or later.