[CI] plugin port should be assignable for tests

We add a new property in `pom.xml` file:

```xml
<es.plugin.port>9500</es.plugin.port>
```

This property can be set by Jenkins so we can run multiple tests in parallel.

Note that each node will use `es.plugin.port + nodeNumber * 10`.
So in a 3 nodes cluster, we thrift will be bound on:

* 9500
* 9510
* 9520

cc @mrsolo

Closes #24.