slap/standalone: Add slapformat configuration

This enables calling `slapos node format` from the command line.

The configuration is updated every time `StandaloneSlapOS.format()`
is called so that subsequent calls to `slapos node format` produce
the same output.

In the future, `StandaloneSlapOS.format()` could be adapted to call
`slapos node format` directly.

See merge request nexedi/slapos.core!308