I have added cache_valid_time=3600 for the first task in each
tag that could be possibly running apt-related commands. For ex,
the "nginx" tag is also in the "packages" tag, but sometimes you
run the nginx tag by itself (perhaps repeatadely), so you'd want
to limit the update unless the cache was 1 hour old
For idempotence we need to run all apt-related tasks, like editing
source files, adding keys, installing packages, etc, when running
the 'packages' tag.
