Added support to sub-url in elasticseach_http output

In several cases I had to work with elasticsearch that is not accessible from the root URL of a server, but from a sub-url (for example, if elasticsearch URL is http:/localhost:9200/elasticsearch/).
I have modified the elasticsearch_http output to support that scenario, by adding a new parameter named path, usually set to empty string, but provides the ability to add sub-url after the port.