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

Reverts: 440a0b40

 ('initrd: set ipv6.method=auto when the autoconfiguration field is 'none'')
Signed-off-by: Antonio Cardace <acardace@redhat.com>