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

I do not think that it's the beast idea in general to limit python package, because:
- those older version could have security issues
- it can cause some other package to fail, because it will required newer version
- we can hit an issue when updating them again

However the charm-telegraf has lot of old dependencies (it's "nice" to see lines like
`# Support for python3.4 (Trusty)`).

What we can do if you really need to have it in main branch is to move these changes to
separate branch, but instead of releasing to `xenial/...` we will release it to `latest`
and immediately build +release version from master branch. Like this the newest version
can be hidden behind `latest`. This will bring more work with release (releasing, promoting
always two version), but if I'm correct there will no much contribution to these charm due
migration to cos.

I'll open discussion in our team about this and I'll let you know.