agent: Error out if IPv4 allocation CIDR is not configured and IPv4 is enabled
agent: Error out if IPv4 allocation CIDR is not configured and IPv4 is enabled

It's always automatically generated when IPv4 is enabled but fail if it is not
set in case the automatic generation is ever disabled.

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

cilium/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,cilium-team/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium