Modify persisted paths by default

[davidcassany]

This is issue is related with #2224. I think we should not make /etc/systemd persisted by default, in fact I think only the bare minimum should be made persistent by default. Ideally we should encourage to set /etc change dynamic as part of the cloud-config setup, so we make the system as stateless as possible.

Changing persistent defaults brings compatibility concerns about how to handle those questions, hence this issue at the end is more about how to handle those rather than applying the change itself.

Should we make mount setup an install choice and stick to that on upgrades? other ideas?