Re: [rhinstaller/blivet/pulls/434 3.0-devel] Configuration actions

[dashea]

I notice this doesn't really do any explicit checking of the new value. I think any errors would come up in `apply`, which I'm not sure is what we want -- I think it might be better to check it from the constructor.

Also, I'm not sure what the value is of setting `self._run` in the action class.

Otherwise I think it looks good.