nixos/version: fix nixops pre 1.6 compatibility

We should be able to deploy a NixOS 18.03 system with the current nixops
stable release. Some options were renamed, so instead of
`mkRenamedOptionModule` we introduce them as read-only interal options
that won't be rendered in the manual.

Only the options that are needed to make nixops evaluations succeed were
added.

This commit should probably be reverted after or before the 18.09 release,
depending on the nixops 1.6 release.

The user will not get the warning that these have been renamed but
this change is mentioned in the release notes.

Fixes #34253.