🔧 Create a configuration file interface

This commit adds the ability to provide a config file in different ways:

The following options are given in the order in which they are checked:
1. A config file path passed by the -c parameter on the command line
2. A config file path passed by the HS_CONFIGFILE environment variable
3. A config.json file in the application root

Should these fail we try to get the parameters for the config struct
directly from environment variables or secret files as used in a
container environment. As example the JwtKey can be supplied by either
passing the value through:

* An environment variable HS_CONFIG_JWTKEY which contains the value directly
* An environment variable HS_CONFIG_JWTKEY_FILE which contains a path
  pointing to the Secret File

auth/auth.go

@@ -15,7 +15,7 @@ func GenerateToken(username string) (string, error) {
 	}
 
 	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
-	tokenString, err := token.SignedString(config.JwtKey)
+	tokenString, err := token.SignedString(config.GlobalConfig.JwtKey)
 
 	return tokenString, err
 }