Allow setting an explicit configuration file path when running the agent

[ema-pe]

Currently, when the DFaaS agent starts, it reads the configuration file (dfaasagent.env) from a fixed absolute directory called /agent:

dfaas/dfaasagent/agent/agent.go

Lines 233 to 234 in 3a9fc2b

	// Load configuration
	_config, err := config.LoadConfig("/agent")

Then, in LoadConfig of the config package, it reads the dfaasagent.env file:

dfaas/dfaasagent/agent/config/config.go

Lines 63 to 79 in 3a9fc2b

	func LoadConfig(path string) (config Configuration, err error) {
	viper.AddConfigPath(path)
	viper.SetConfigName("dfaasagent")
	viper.SetConfigType("env")
	viper.SetEnvPrefix("AGENT")
	viper.AllowEmptyEnv(true)
	viper.AutomaticEnv()
	err = viper.ReadInConfig()
	if err != nil {
	return
	}
	err = viper.Unmarshal(&config)
	return
	}

It would be better to tell the agent where to read the configuration file, using the default path only as a fallback.

[miciav]

I disagree; having a configuration file in a fixed directory is common practice.
I would work in the direction to use also environment variables for a greater flexibility.

[ema-pe]

I disagree; having a configuration file in a fixed directory is common practice. I would work in the direction to use also environment variables for a greater flexibility.

I agree to use environment variables (and the config files that contain them). But I would add a command line option to just specify the path to an additional config file that overwrites the contents of the default one.

Now, if the agent doesn't find the configuration file in the fixed directory, it won't print any warnings or errors, resulting in a useless configuration like the one shown here. This can lead to some unpredictable behavior that is hard to understand. I would change this in two ways:

1. Add default values (taken from dfaasagent.env here) as a configuration file to be embedded or in config.go using the Viper API,
2. Print the path of all configuration files read during startup in debug mode to know where and which files the agent is reading.

[aletundo]

I also think that despite it's a common practice to have a fixed default directory where to load the env file / config file, it's good to have a parameter that can override the default location, which should be used only as default fallback (and the agent should or fail to start if no config file is found, or use all the default values / file with a warning).