lxc-user-nic should support DHCP Renew

[AkihiroSuda]

panic: expected to retain 10.0.3.206, got 10.0.3.207

goroutine 6 [running]:
github.com/rootless-containers/rootlesskit/pkg/network/lxcusernic.dhcpRenewRoutine(0xc000190fc0, 0xc000029080, 0x4, 0xc000029a2c, 0x4, 0x4, 0x34630b8a000)
        /home/suda/gopath/src/github.com/rootless-containers/rootlesskit/pkg/network/lxcusernic/lxcusernic.go:147 +0x2d3
created by github.com/rootless-containers/rootlesskit/pkg/network/lxcusernic.(*childDriver).ConfigureNetworkChild
        /home/suda/gopath/src/github.com/rootless-containers/rootlesskit/pkg/network/lxcusernic/lxcusernic.go:129 +0x82a
[rootlesskit:parent] error: child exited: exit status 2

https://github.com/rootless-containers/rootlesskit/blob/v0.9.4/pkg/network/lxcusernic/lxcusernic.go#L133-L151