Do not consider environment variables from viper for CLI commands

[Gerrit91]

At the moment, a user can export env variables, which will then be available through viper and impact a lot of code, like:

// from admin/v1/tenant.go
...
	if viper.IsSet("admitted") {
		req.Admitted = pointer.Pointer(viper.GetBool("admitted"))
	}
	if viper.IsSet("limit") {
		req.Paging = &apiv1.Paging{
			Count: pointer.Pointer(viper.GetUint64("limit")),
			Page:  nextpage,
		}
	}
	if viper.IsSet("provider") {
		return nil, fmt.Errorf("unimplemented filter by provider")
	}

So if a user exports METAL_STACK_CLOUD_ADMITTED it will influence the output of the command, which I do not think should be possible. In this example it is maybe not so dangerous but for deletion or update commands this could lead to unexpected problems. We should therefore only consider --admitted parsed directly from the command line.