host-local: allow ip request via CNI_ARGS

A specific IP can now be requested via the environment variable CNI_ARGS, e.g.
`CNI_ARGS=ip=1.2.3.4`.
The plugin will try to reserve the specified IP.
If this is not successful the execution will fail.

plugin/args.go

@@ -0,0 +1,36 @@
+package plugin
+
+import (
+	"encoding"
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+func LoadArgs(args string, container interface{}) error {
+	if args == "" {
+		return nil
+	}
+
+	containerValue := reflect.ValueOf(container)
+
+	pairs := strings.Split(args, ",")
+	for _, pair := range pairs {
+		kv := strings.Split(pair, "=")
+		if len(kv) != 2 {
+			return fmt.Errorf("ARGS: invalid pair %q", pair)
+		}
+		keyString := kv[0]
+		valueString := kv[1]
+		keyField := containerValue.Elem().FieldByName(keyString)
+		if !keyField.IsValid() {
+			return fmt.Errorf("ARGS: invalid key %q", keyString)
+		}
+		u := keyField.Addr().Interface().(encoding.TextUnmarshaler)
+		err := u.UnmarshalText([]byte(valueString))
+		if err != nil {
+			return fmt.Errorf("ARGS: error parsing value of pair %q: %v)", pair, err)
+		}
+	}
+	return nil
+}