feat(config): default api.targets.auto_discover to true

BREAKING CHANGE: Change the default value of
`api.targets.auto_discover`, which [automatically detects sources and
layers from Elasticsearch](https://github.com/pelias/api#custom-sources-and-layers), to true.

This setting is the generally recommended value and is required for
Pelias users importing their own custom data.

config/defaults.json

@@ -38,7 +38,7 @@
     "indexName": "pelias",
     "version": "1.0",
     "targets": {
-      "auto_discover": false,
+      "auto_discover": true,
       "canonical_sources": ["whosonfirst", "openstreetmap", "openaddresses", "geonames"],
       "layers_by_source": {
         "openstreetmap": [ "address", "venue", "street" ],

test/expected-deep.json

@@ -43,7 +43,7 @@
     "host": "http://pelias.mapzen.com/",
     "version": "1.0",
     "targets": {
-      "auto_discover": false,
+      "auto_discover": true,
       "canonical_sources": ["whosonfirst", "openstreetmap", "openaddresses", "geonames"],
       "layers_by_source": {
         "openstreetmap": [ "address", "venue", "street" ],