Switched to entrypoint instead of cmd

When switching from CMD to ENTRYPOINT it will be easy to add command line arguments to elasticsearch without knowing the location of the elasticsearch binary inside the container. Running this container without arguments will stay the same
`docker run -d -p 9200:9200 -p 9300:9300 dockerfile/elasticsearch`

and running with arguments will be 
`docker run -d -p 9200:9200 -p 9300:9300 dockerfile/elasticsearch -Des.network.publish_host=192.168.59.103`