[FIX] Make the "install_plugin" definition idempotent
[FIX] Make the "install_plugin" definition idempotent

(Due to the change in ES 0.19, the old way of testing plugin presence was incorrect.)

gilt/cookbook-elasticsearch,zzugg/cookbook-elasticsearch,spuder/cookbook-elasticsearch,zzugg/cookbook-elasticsearch,vacay/vcy-elasticsearch,spuder/cookbook-elasticsearch,nbudin/cookbook-elasticsearch,michaeld/elasticsearch-cookbook,GemHQ/cookbook-elasticsearch,vacay/vcy-elasticsearch,gilt/cookbook-elasticsearch,gouketsu/cookbook-elasticsearch,paulczar/cookbook-elasticsearch,vacay/vcy-elasticsearch,spuder/cookbook-elasticsearch,bavardage/cookbook-elasticsearch,nbudin/cookbook-elasticsearch,gouketsu/cookbook-elasticsearch,nbudin/cookbook-elasticsearch,zzugg/cookbook-elasticsearch,gouketsu/cookbook-elasticsearch,gilt/cookbook-elasticsearch