Add config log format validation

[krhubert]

close #1387

[NicolasMahe]

@krhubert Don't forget to remove the now useless test lines L163-R165:

engine/config/config.go

Lines 163 to 165 in 43514a9

	if !xstrings.SliceContains([]string{"text", "json"}, c.Log.Format) {
	return fmt.Errorf("config.Log.Format value %q is not an allowed", c.Log.Format)
	}