Improve ckan 2.10 (and higer) compatibility: config_option declaration

[PitPitoune]

Hi the team,
As adviced here: doc ckan config declaration
I suggest to declare your config options.
Without that, I (and all) have warnings every time we rebuild SOLR indexes (and 1 warning for each dataset)

Easy way to do that (as suggested in the doc): add the line "@toolkit.blanket.config_declarations" in the plugin.py and, in the same path, add a file 'config_declaration.yaml' as described.

Thanks a lot in advance