River does not start when using config/templates files

From elasticsearch 0.90.6, when you have templates files defined in `config/templates` dir, rivers don't start anymore.

Steps to reproduce:

Create `config/templates/default.json`:

```javascript
{
  default:
  {
    "template" : "*",
    "mappings" : {
      "_default_" : {
      }
    }
  }
}
```

Start a dummy river:

```sh
curl -XPUT 'localhost:9200/_river/my_river/_meta' -d '{ "type" : "dummy" }'
```

It gives:

```
[2014-01-01 22:08:38,151][INFO ][cluster.metadata         ] [Forge] [_river] creating index, cause [auto(index api)], shards [1]/[1], mappings [_default_]
[2014-01-01 22:08:38,239][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:38,245][INFO ][cluster.metadata         ] [Forge] [_river] update_mapping [my_river] (dynamic)
[2014-01-01 22:08:38,250][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:39,244][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:39,252][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:40,246][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:40,254][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:41,246][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:41,255][INFO ][river.routing            ] [Forge] no river _meta document found, retrying in 1000 ms
[2014-01-01 22:08:42,249][WARN ][river.routing            ] [Forge] no river _meta document found after 5 attempts
[2014-01-01 22:08:42,257][WARN ][river.routing            ] [Forge] no river _meta document found after 5 attempts
```

With elasticsearch 0.90.2 or with no template file in `config/templates` dir, it gives:

```
[2014-01-01 22:22:32,096][INFO ][cluster.metadata         ] [Forge] [_river] creating index, cause [auto(index api)], shards [1]/[1], mappings []
[2014-01-01 22:22:32,221][INFO ][cluster.metadata         ] [Forge] [_river] update_mapping [my_river] (dynamic)
[2014-01-01 22:22:32,228][INFO ][river.dummy              ] [Forge] [dummy][my_river] create
[2014-01-01 22:22:32,228][INFO ][river.dummy              ] [Forge] [dummy][my_river] start
[2014-01-01 22:22:32,234][INFO ][cluster.metadata         ] [Forge] [_river] update_mapping [my_river] (dynamic)
```

Closes #4577.