plugins/apt_all: Avoid using an external cron job

As commented by mtillberg in his ticket :

	apt_all update cron job is no longer necessary.

	During fetch, plugin state timestamp is checked against apt cache and
	dpkg state files to determine if an update is required.

	Plugin won't run if the apt cache is not available to prevent download.

	Plugin can run as nobody and use $ENV{MUNIN_PLUGSTATE}.

	Note that with this update, the built-in daily apt periodic check and/or update
	should be used:

	# cat /etc/apt/apt.conf.d/10periodic
	APT::Periodic::Update-Package-Lists "1";
	APT::Periodic::Download-Upgradeable-Packages "0";
	APT::Periodic::AutocleanInterval? "0";
	APT::Periodic::Unattended-Upgrade "0";

	Additionally, any manual operations (apt-get update, package installs)
	will be caught by the plugin at the next fetch.

Closes: #1333