Add option to configure poseidon

It exists a default configuration. It can be overwritten by adding
a configuration.yaml. This can again be overwritten by environment
variables.

config/config.go

@@ -0,0 +1,116 @@
+package config
+
+import (
+	"flag"
+	"fmt"
+	"gopkg.in/yaml.v3"
+	"log"
+	"os"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+var (
+	Config = &configuration{
+		Server: server{
+			Address: "127.0.0.1",
+			Port:    3000,
+		},
+		Nomad: nomad{
+			Address: "",
+			Token:   "",
+			TLS:     false,
+		},
+		Logger: logger{
+			Level: "INFO",
+		},
+	}
+)
+
+type server struct {
+	Address string
+	Port    int
+}
+
+type nomad struct {
+	Address string
+	Token   string
+	TLS     bool
+}
+
+type logger struct {
+	Level string
+}
+
+type configuration struct {
+	Server server
+	Nomad  nomad
+	Logger logger
+}
+
+func InitConfig() {
+	content := readConfigFile()
+	Config.mergeYaml(content)
+	Config.mergeEnvironmentVariables()
+}
+
+func readConfigFile() []byte {
+	var configFilePath string
+	flag.StringVar(&configFilePath, "config", "./configuration.yaml", "path of the yaml config file")
+	data, err := os.ReadFile(configFilePath)
+	if err != nil {
+		log.Printf("Using default configuration... (%v)", err)
+	}
+	return data
+}
+
+func (c *configuration) mergeYaml(content []byte) {
+	if err := yaml.Unmarshal(content, c); err != nil {
+		log.Fatalf("Could not parse configuration: %v", err)
+	}
+}
+
+func (c *configuration) mergeEnvironmentVariables() {
+	readFromEnvironment("POSEIDON", reflect.ValueOf(c).Elem())
+}
+
+func readFromEnvironment(prefix string, value reflect.Value) {
+	if !value.CanSet() || !value.CanInterface() {
+		return
+	}
+
+	if value.Kind() != reflect.Struct {
+		content, ok := os.LookupEnv(prefix)
+		if !ok {
+			return
+		}
+		switch value.Kind() {
+		case reflect.String:
+			value.SetString(content)
+		case reflect.Int:
+			integer, err := strconv.Atoi(content)
+			if err != nil {
+				log.Printf("Could not parse environment variable %s with value '%q' as integer", prefix, content)
+				return
+			}
+			value.SetInt(int64(integer))
+		case reflect.Bool:
+			boolean, err := strconv.ParseBool(content)
+			if err != nil {
+				log.Printf("Could not parse environment variable %s with value '%q' as boolean", prefix, content)
+				return
+			}
+			value.SetBool(boolean)
+		default:
+			// ignore this field
+			log.Printf("Setting configuration options of type %s via environment variables is not supported", value.Type().Name())
+		}
+	} else {
+		for i := 0; i < value.NumField(); i++ {
+			fieldName := value.Type().Field(i).Name
+			newPrefix := fmt.Sprintf("%s_%s", prefix, strings.ToUpper(fieldName))
+			readFromEnvironment(newPrefix, value.Field(i))
+		}
+	}
+}