initrd: set required-timeout by default for IPv4 configuration

If the kernel command-line doesn't contain an explict ip=$method, currently the generator creates connections with both IPv4 and IPv6 set to 'auto', and both allowed to fail.

Since NM is run in configure-and-quit mode in the initrd, NM can get an IPv4 address or an IPv6 one (or both) depending on which address family is quicker to complete. This unpredictable behavior is not present in the legacy module, which always does IPv4 only by default.

Set a required-timeout of 20 seconds for IPv4, so that NM will preferably get an IPv4, or will fall back to IPv6.

https://bugzilla.redhat.com/show_bug.cgi?id=1961666 #729 (closed)