validate that ipv6_disabled is conflicting with other settings
validate that ipv6_disabled is conflicting with other settings

When IPv6 is disabled, it is conflicting with enabling `auto6`
or configuring static IPv6 or configuring `gateway6` or
configuring `route_metric6`. Add unit tests to check if
this is properly handled in Argvalidator.

Signed-off-by: Wen Liang <5ee3357fb455f67019b3676c8968488c41307286@gmail.com>

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network