dracut: add slow DHCP4 tests
- add nfs_ip6 bridge which is ipv6 only
- delay DHCP4 response in dhcpd.conf for specific MAC
- remove some kernel params (rd.retry, rd.net.tiemout.dhcp...), tests should pass without it being specified
- add 3 tests covering: https://bugzilla.redhat.com/show_bug.cgi?id=1961666