Updated plugin docs for 2.1.0

Merge pull request elastic#13894 from dmland/patch-1

Update query-string-syntax.asciidoc

[TEST] Add test for elastic#14232

Test that ClassSet bindings bound as eager singletons

release [1.7.3]

Update version incompatibility message for plugin manager

When the plugin manager does not find in `plugin-descriptor.properties` the exact same elasticsearch version it was built on
as the current elasticsearch version, it fails with a message like:

```
ERROR: Elasticsearch version [2.0.0-beta1] is too old for plugin [elasticsearch-mapper-attachments]
```

Actually, the message should be:

```
Plugin [elasticsearch-mapper-attachments] is incompatible with Elasticsearch [2.0.0.beta2]. Was designed for version [2.0.0.beta1].
```

The opposite is true. If you try to install a version of a plugin which was built with a newer version of elasticsearch, it will fail the same way:

```
Plugin [elasticsearch-mapper-attachments] is incompatible with Elasticsearch [2.0.0.beta1]. Was designed for version [2.0.0.beta2].
```

(cherry picked from commit 28f82fb)
(cherry picked from commit 9886abd)
(cherry picked from commit 6138450)

Release script: Improve automation for package repositories

* Automatic package repository creation for debian and rpm repositories using deb-s3 and rpm-s3 tools
* Fixing paths in email for repositories
* Add manual verification step for maven staging repo
* Do not create release directory in /tmp, because we might loose it on VMs
* Removed unused download-s3 script
* Add signage check for RPM
* Removed download-s3.py/upload-s3.py, as they are unused

Closes elastic#13209

release [1.7.2]

Tag version 2.0.0-beta1

release [1.7.1]

release [1.6.2]