run Puppet agent from systemd only after network is brought up

Using After ensures proper ordering between the Puppet service and
the machine's networking. We could also have used Wants or
Requires but, according to the systemd.unit(5) manpage, After
only ensure *ordering*, while Wants and Requires
actually *trigger* units.

In other words, After ensures that puppet-run will be started
after networking is up, but will not *necessarily* run when networking
is up, nor will it attempt to bring up networking when ran.

Closes: #764