Vendor not read from .drackvmrc

[radek-senfeld]

Hello,
thank you once more for this marvelous tool! Today I started using it for Supermicro as well.

There's a minor annoyance with --vendor having default value set to "dell". It causes ignoring vendor set in .drackvmrc.

The reason is this if block - *_vendor is never an empty string due to the default value. (https://github.com/rockyluke/drac-kvm/blob/master/main.go#L141)

	if *_vendor == "" {
		if value, err := cfg.GetValue(*_host, "vendor"); err == nil {
			vendor = value
		} else {
			// To keep old default behaviour we set vendor string to dell by default.
			vendor = "dell"
		}
	} else {
		vendor = *_vendor
	}

	if _, err := kvm.CheckVendorString(vendor); err != nil {
		log.Fatalf("Provided vendor: %s, is not supported consider adding support with Github PR...", vendor)
	}

Quick and dirty fix is to replace if *_vendor == "" { with if *_vendor == "" || *_vendor == "dell" { but it's clearly suboptimal.