[build] mark elasticsearch as provided in plugins
When we build a plugin, we suppose it will be executed within elasticsearch server.
So we should mark it as `provided`.
If a java developer needs to embed the plugin and elasticsearch, it will make sense to declare both in its `pom.xml` file.
In Maven parent project, in dependency management, we should only declare which versions of 3rd party jars we want to use but not force any scope.
It makes then more obvious in modules what is exactly the scope of any dependency.
For example, one could imagine importing `jimfs` as a `compile` dependency in another module/plugin with:
```xml
<dependency>
<groupId>com.google.jimfs</groupId>
<artifactId>jimfs</artifactId>
</dependency>
```
But it won't work as expected as the default maven `scope` should be `compile` but here it's `test` as defined in the parent project.
So, if you want to use this lib for tests, you should simply define:
```xml
<dependency>
<groupId>com.google.jimfs</groupId>
<artifactId>jimfs</artifactId>
<scope>test</scope>
</dependency>
```
We also remove `maven-s3-wagon` from gce plugin as it's not used.
When we build a plugin, we suppose it will be executed within elasticsearch server.
So we should mark it as `provided`.
If a java developer needs to embed the plugin and elasticsearch, it will make sense to declare both in its `pom.xml` file.
David Pilato
Simon Willnauer