slap/standalone: Enable slapos node format

Enable slapos node format in StandaloneSlapOS configuration:

• add a slapformat-definition.cfg file
• add a [slapformat] section in slapos.cfg referencing slapformat-definition.cfg

Formatting in standalone is handled by StandaloneSlapOS.format() in an independent way that doesn't require these configuration files.

Now, calling StandaloneSlapOS.format() also updates these configuration files so that subsequent calls to slapos node format should produce the same outcome.

In the future, StandaloneSlapOS.format() and slapos node format could be further unified by making StandaloneSlapOS.format() call slapos node format as part of its implementation.

Also, fix a spurious slapos node format warning.