IP Address validator

Returns True if there is at most one occurence of the ip_interface (an IPv4/IPv6 prefix+len)
in the entire config. That said, we need the 'iface_addresses' because VPP is a bit fickle in
this regard.

IP addresses from the same prefix/len can be added to a given interface (ie 192.0.2.1/24 and
192.0.2.2/24), but other than that, any prefix can not occur as a more-specific or less-specific
of any other interface.

So, we will allow:
- any ip_interface that is of equal network/len of existing one(s) _on the same interface_

And, we will reject
- any ip_interface that is a more specific of any existing one
- any ip_interface that is a less specific of any existing one

Update unit tests to ensure ip_interfaces are allowed in all cases.

unittest/error-subinterface5.yaml

@@ -13,5 +13,5 @@ interfaces:
     lcp: "xe2-0-0"
     sub-interfaces:
       100:
-        addresses: [ 192.0.2.1/24 ]
+        addresses: [ 192.168.1.1/24 ]