Add support for IPv4LL start-address

[troglobit]

Infix use avahi-autoipd to provide IPv4LL service. It comes with a --start=ADDR option which in many cases would be extremely useful to set up predictable addresses on shared links.

• [x] Extend infix-ip.yang model with an autoconf start-addres IPV4ADDRautoconf request-address IPV4ADDR setting
• [x] Update confd to set avahip-autoipd start address
• [x] Add regression test case

[troglobit]

Changed syntax from start-address to request-address, which seemed more appropriate.