Use systemd for disabling swap when it's used (#10587)

* Mask systemd swap.target do disable swap

This is a more generic way to disable swap, since it pulls .swap units
in systemd distributions; fstab is only one way to generate .swap units.

* Unconditionally disable swap

We only care to disable it (the "swapon" registered variable is not used
anywhere else.
This allows to get rid of the ignore_errors, since this was added
because swapon.stdout does not exist in check_mode (see issue #6642).

* Don't explicitly disable swapOnZram

We're already masking the swap.target, which would pull the zram unit,
hence no need to handle zram-generator specifically.