initrd: disable ipv6 when 'off|none' is set in the 'ip' option
This is potentially a breaking change, formerly speciyfing 'none|off' in the kernel cmdline option 'ip' was understood by the dracut network-module as doing 'ipv6.method=auto' which is clearly incosistent with the 'off' naming, thus 'off|none' now means to actually disable both ipv6 and ipv4 (unless a static ip is provided).
Unit test added.
https://bugzilla.redhat.com/show_bug.cgi?id=1883958
Signed-off-by: Antonio Cardace acardace@redhat.com