rpm,deb: make ES user own plugins dir, remove on uninstall

This change will chown /usr/share/elasticsearch/plugins to the elasticsearch user (the directory was formerly owned by root). This enables the ES user to manage plugins.

Also, please note that /usr/share/elasticsearch/plugins **is now removed when the elasticsearch package is un-installed**. Previously it was left lying there.

Fixes #8419, tested with Fedora 21 (RPM) and Debian 7 (DEB).