ncm-metaconfig: support for filename attribute to force the filename instead the service key

Support for defining the filename allows for generated filenames and typical shorter/human readable paths in the templates e.g. 
```
"/software/components/metaconfig/services/{/etc/httpd/conf.d/ssl.conf}/contents" = "foo"
```
could become
```
variable NAME = "ssl";
"/software/components/metaconfig/services/vhost/filename" = format("/etc/httpd/conf.d/%s.conf", NAME);
"/software/components/metaconfig/services/vhost/contents" = "foo"
```