[CI] plugin port should be assignable for tests

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

```xml
<es.plugin.port>11211</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, memcached will be bound on:

* 11211
* 11221
* 11231

cc @mrsolo

Closes #24.

(cherry picked from commit da51c01283d7a71a49ebf7d083cf36c678763705)