This commit removes the system key from master and changes watcher to use a secure setting instead
for the encryption key.
Original commit: elastic/x-pack-elasticsearch@5ac95c60ef
* [DOCS] Move security APIs to Elasticsearch Ref
* [DOCS] Update links to security APIs
* [DOCS] Fix link to security APIs
Original commit: elastic/x-pack-elasticsearch@d7a9d3f1ab