Code review comment for lp:~raharper/curtin/trunk.fix-lp1722322

The dirty disks mode we have just lets the existing config mode pass through.
We can just as easily call curtin-blockmeta with an arbitrary config.

It was just easiest (and immediately effective) to inherit the config from the install environment.

it'd be easy enough to make the 'Injecting early_command to dirty storage devices' code that is in vmtests/__init__.py accept 'cls.dirty_disks' be a config itself or a path to one (rather than just boolean). Then generate a portion of the config that would write the different config.