Add os-networks JSON client for tempest

There is no client for the os-networks API.
Scenario tests do use that API, and migrating them to tempest clients
triggered the need for this client to exists.
Scenario tests only run with JSON, so only providing JSON implementation.

Partially implements bp:tempest-client-scenarios

Change-Id: I85da97a98fc558af14e2a9fc019b6e712653ecf2
Merge "Add os-networks JSON client for tempest"