Fix #273; Add 1.4 LS_HEAP_SIZE support

logstash 1.4 was only getting 500mb heap size from /opt/logstash/server/bin/logstash since LS_HEAP_SIZE was not set. This rectifies that with a default of 60% available total memory.

Also, chefspec wasn't passing because of logstash_service[name] mismatch in issue#273; the service_name change in config.rb provider seems to take care of that.