Packaging: 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, /usr/share/elasticsearch/plugins is now removed when the elasticsearch
package is un-installed. Previously it was left lying there.

Closes #8732

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>