plugins/host-local: ensure subnet is a network address

Allocation code assumes the specified subnet is a clean network address
prefix, so check that is the case and throw an error otherwise

Fixes #161

plugins/ipam/host-local/backend/allocator/config_test.go

@@ -372,7 +372,7 @@ var _ = Describe("IPAM config", func() {
 					"type": "host-local",
 					"ranges": [
 						[{"subnet": "10.1.2.0/24"}],
-						[{"subnet": "2001:db8:1::/24"}]
+						[{"subnet": "2001:db8:1::/48"}]
 					]
 				}
 			}`