add reading the SecretKey from environment variables.

config:
```
	"aliases": {
		"default": {
			"url": "http://127.0.0.1:9000",
			"accessKey": "minioadmin",
			"secretKey": "$MINIO_SECRET",
			"api": "s3v4",
			"path": "auto"
		},
```
NewS3Config:
```
		s3Config.SecretKey = aliasCfg.SecretKey
		secretEnv := os.ExpandEnv(aliasCfg.SecretKey)
		if secretEnv != "" {
			s3Config.SecretKey = secretEnv
		}
```