Add optional notify_update parameter

[clauded]

In our environment we make sure apt_update is ran before any packages is installed with the following:
Exec['apt_update'] -> Package <| |>

With this statement, a dependency cycle is created:
(Exec[apt_update] => Package[unattended-upgrades] => Apt::Conf[unattended-upgrades] => Apt::Setting[conf-unattended-upgrades] => File[/etc/apt/apt.conf.d/50unattended-upgrades] => Class[Apt::Update] => Exec[apt_update])

It's possible to call apt::conf (v2.2.0 or later) and not run apt_update by specifying 'notify_update => false'. This avoids our dependency problem.

This PR adds an optional parameter 'notify_update' to init.pp (if the parameter is not specified, the defaults applies and apt_update is notified). The parameter can be undef (default), true or false.

[igalic]

what does this mean for our own version?

[ffrank]

I don't believe our dependencies have any bearing on semver progression. If this change requires a bump, we bump. Updates of our version are just determined by the nature of the change itself.

[ffrank]

Hmm given more thought, I'm not so sure anymore. But a minor update in a dependency should not cause issues for (m)any users.

[clauded]

A user upgrading this module without upgrading the apt module (using r10k for example) will get an error at execution (invalid parameter). The change should be well documented in the changelog.

[chrisjohnson1988]

I have hit this issue to. I got around it by adding:

Apt::Conf<|tag=='unattended-upgrades'|> {
    notify_update => false
}

in our manifest which include unattended-upgrades. A little obscure, so +1 for this pull request.