Check for packaged config file in plugin install script

In case you are running the plugin command from a packaged installation
like the debian or RPM package, a change of the path.plugins directory
in the elasticsearch.yml directory was ignored, because it was not loaded.

This change checks for the existence of /etc/elasticsearch/elasticsearch.yml
and the absence of $ES_HOME/config/elasticsearch.yml - in that case a package
installation is assumed and the -Des.default.config parameter is appended to
the JAVA_OPTS if it does not exist yet.

Closes #3304