[STORM] Allow declarative fields for EsSpout

The EsSpout can be configured to 'unwrap' the retrieved document
directly into the target tuple. Unfortunately due to Storm's lifecycle
component, the configuration cannot be read from Elastic and needs to
be specified manually.

Fix #282 #286

EsSpout to declare real fields #282

Signed-off-by: Costin Leau <costin.leau@gmail.com>