Apparently, `puppet-lint` on Ubuntu trusty throws warnings for certain
quoting patterns that are OK in modern `puppet-lint`. I believe the
old Zulip code was actually correct (i.e. the old `puppet-lint`
implementation was the problem), but it seems worth changing anyway to
suppress the warnings.
We also exclude more of puppet-apt from linting, since it's
third-party code.
We start to use puppet-lint to lint puppet modules by default by
adding it to tools/lint (which controls our linter tool chain).
We also define a few puppet-lint rules to exclude.
Fixes: #9185.