Split service instance into networking and compute parts

Service instance module now creates a server for shares and also configuring network for this server. But manila should work using not only neutron network, but also nova network. So it is good idea to split service instance module into 2 parts Service instance which will have all server-creating tasks and network helpers which will have all network related methods.
For this it is necessary to create sub-modules for service instance (let's call it network helpers) which will store all network functionality of service instance. Each submodule should works with some openstack network component. So for now there will be 2 network helpers. For working with neutron network and nova network.