Use __FILE__-relative path for the default template.

Users reported this (or similar) error:
Errno::ENOENT: No such file or directory -
lib/logstash/outputs/elasticsearch/elasticsearch-template.json

This is because the file being searched-for was relative to $PWD, not
relative to the elasticsearch output plugin file itself.

Tested works like this:

    % logstash/bin/logstash -e 'output { elasticsearch {  embedded => true } }'

Prior to this patch, the above command line would fail with this error:

    Errno::ENOENT: No such file or directory - lib/logstash/outputs/elasticsearch/elasticsearch-template.json
    ...
        get_template at /home/jls/projects/logstash/lib/logstash/outputs/elasticsearch.rb:285