fedora/centos: do no install systemd and passwd by default
fedora/centos: do no install systemd and passwd by default

There is no reason to require passwd to be installed. Depending on
what the image is used for, user logins through with traditional
authentication might be completely unnecessary. Similarly, systemd is
only useful when the image is to be booted (under qemu or as a
container). But when args.bootable is true, systemd-udev will pull in
systemd anyway. So let's just reduce the packages we always pull in to
the minimum that defines each distro. Any additional packages should
be pulled in through the project configuration.

UbiCastTeam/mkosi,UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi