Fix @persistent@ substitution logic.

Just handle @persistent@ as a normal boolean.
CC @dbent

this was already there in `systemd-cron` before the other project `systemd-crontab-generator` got merged in; this is needed for the static units like `cron-daily.timer`

the assumption of the two project were different; one required systemd > 197 and ther other systemd > 212, this is the comprise I found to let the generator work with < 212.

In the original generator, all the units were unconditionally generated with `"Persistent=true"`
https://github.com/kstep/systemd-crontab-generator/commit/9ef23039154a5360fd161506ba687c850a96c3b1

I *do* remember having a lot of warning for this in the journal, so I just commented it (`#Persistent=true"`) in my working tree. At his moment I had no experience of python, I was just writing man pages, so It wasn't a problem. I just kept ignoring it in my `git diff`

You can dig a old systemd on http://snapshot.debian.org/ if you want to test it.

We don't want to drop support for systemd < 212 for now ( https://github.com/systemd-cron/systemd-cron/issues/29#issuecomment-60937019 )