Code review comment for ~hloeung/charm-telegraf:master

Hi @hloung, you are right that Xenial is supported via Expanded Security Maintenance (ESM),
but I do not think that it's good idea to restore it to main branch. Why i think so is
because, in charmhub.io it will be visible as supported, what is not really true, because
only security patches should be supported and that should be done via separate branch and
channel. Like `xenial` and `xenial/stable`.

Is this patch fixing some critical issues for Xenial? If not what is reason to do it, since
you can still do `juju deploy telegraf --series xenial` and you will get revision 49. If yes,
I would suggest to create Xenial branch as I mention before.