[slapos_hypermedia] Provide link to Instance's Hosting Subscription when doing Instance query.
[slapos_hypermedia] Test Instance scenario.
[slapos_hypermedia] Test Instance login and getting Hosting Subscription from Instance.
[SlapOS Product] add _makeComputerNetwork helper to testSlapOSMixin.

Also return created objects for _createComputer.
[slapos_cloud] Add support for "unique_by_network" SLA mode.