initrd: generate ipv6.method=auto for ip=dhcp6

When a 'ip=auto6' option is passed to kernel, the old dracut network
module only sets accept_ra in kernel and wait for the address to
appear. Instead, with a 'ip=dhcp6' option it starts 'dhclient -6',
leaving accept_ra to the initial value (that is already 1). So
'ip=dhcp6' in practice does kernel IPv6 autoconf and DHCPv6 at the
same time, without honoring the 'Managed' flag of the router
advertisement.

It seems that the only reason to have distinct 'auto6' and 'dhcp6'
options was that network module did not support starting DHCPv6 only
when necessary based on the M flag of the RA; so the user had to
specify if DHCPv6 was needed or not.

Given that 1) NM is smarter and can start DHCPv6 only when needed by
RA; 2) DHCPv6 alone only gets a /128 address without a prefix route
and so it's not useful; then it makes sense to generate a connection
with 'ipv6.method=auto' for both 'ip=auto6' and 'ip=dhcp6'.

https://bugzilla.redhat.com/show_bug.cgi?id=1854323
!571
21 jobs for bg/initrd-dhcp6-rh1854323 in 36 minutes and 10 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #3526666
checkpatch

00:01:17

manual #3526673
allowed to fail manual
t_centos:7.5.1804
manual #3526674
allowed to fail manual
t_centos:7.6.1810
manual #3526675
allowed to fail manual
t_centos:7.7.1908
manual #3526676
allowed to fail manual
t_centos:7.8.2003
manual #3526677
allowed to fail manual
t_centos:8.1.1911
manual #3526683
allowed to fail manual
t_debian:10
manual #3526682
allowed to fail manual
t_debian:9
manual #3526685
allowed to fail manual
t_debian:sid
manual #3526684
allowed to fail manual
t_debian:testing
manual #3526667
allowed to fail manual
t_fedora:28
manual #3526668
allowed to fail manual
t_fedora:29
manual #3526669
allowed to fail manual
t_fedora:30
passed #3526670
t_fedora:31

00:36:10

manual #3526671
allowed to fail manual
t_fedora:32
manual #3526672
allowed to fail manual
t_fedora:rawhide
manual #3526678
allowed to fail manual
t_ubuntu:16.04
manual #3526679
allowed to fail manual
t_ubuntu:18.04
manual #3526681
allowed to fail manual
t_ubuntu:devel
manual #3526680
allowed to fail manual
t_ubuntu:rolling
 
  External
canceled https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1649/

310:10:21