Add support for cache layout templates

[traviskirstine]

Allow uses to define the template used to reference (key) a tile within a defined cache. This would be applicable to caches that do not follow a defined specification such as file, riak, couch, redis, azureblob and s3 caches. This could allow users to connect directly to seeded caches build with other applications (e.g. MapCache). Similar to the method employed by Mapcache key_template and template

Example:

caches:
   my_cache:
      grids: [my_grid]
      sources: [my_source]
      format: mixed
      request_format: image/vnd.jpeg-png
      cache:
         type: riak
         bucket: my_bucket
         key_template:  X_Y_Z 
         nodes:
            - host: riak.my_server.com
         coverage:
            bbox: [-79.80047550786563, 43.73183855591305, -79.13107337870134, 44.399298533570544]
            srs: 'EPSG:4326'