implement trivial opengeometadata route to send metadata files

rom a cloned repo in `/var/cache/opengeometadata`. related to geoblacklight/geoblacklight#104
Can we dry this up by using `metadata_format` as an optional parameter?

```
get 'opengeometadata/layers/:institution/:layer_id
```
By then requesting `/opengeometadata/layers/stanford/layer-id?metadata-format="fgdc"`