common: Refactor LinuxKeyringSecret into Keyring Interface
Goal: Support multiple backends and faking / mocking for testing.
Add abstract classes Keyring (factory) and KeyringSecret. Add
"Unsupported" implementation for non-Linux platforms. Add a get_best
factory function that currently returns the LinuxKeyring impl on Linux
or Unsupported elsewhere.
Signed-off-by: Marcel Lauhoff <marcel.lauhoff@clyso.com>
On-behalf-of: SAP marcel.lauhoff@sap.com