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>
[TESTS] Use a _default_ template to load field data lazily

Previously it was possible for the field data clearing in this test to
take too long, causing the test to time out.

This also switches to using `scaledRandomIntBetween` for the number of
fields.
[TESTS] Log what the _default_ template is in ElasticsearchIntegrationTest
[TEST] wait unitl all machines joined the cluster