This commit changes work done in #60 by using automatic store type discovery which exists in elasticsearch core.

Instead of writing configuration like:

```yaml
index.store.type: org.elasticsearch.index.store.fs.SmbMmapFsIndexStoreModule
index.store.type: org.elasticsearch.index.store.fs.SmbSimpleFsIndexStoreModule
```

We can now use:

```yaml
index.store.type: smb_mmap_fs
index.store.type: smb_simple_fs
```

Note that we move `org.elasticsearch.index.store.fs.SmbMmapFsIndexStoreModule` to  `org.elasticsearch.index.store.smbmmapfs.SmbMmapFsIndexStoreModule` and `org.elasticsearch.index.store.fs.SmbSimpleFsDirectoryService` to `org.elasticsearch.index.store.smbsimplefs.SmbSimpleFsDirectoryService` instead of deprecating the old classes and add new ones.
It means that if users were using a 2.5.2-SNAPSHOT version, they will need to update their settings.

(cherry picked from commit 613ce5a)
(cherry picked from commit 7831521)

Closes elastic#60.

…allation

Define a proxy for plugin installation

With this commit we don't only make the query index prefix overridable
but also the actual index pattern (with the track parameter
`query_index_pattern'.

With this commit we treat x-pack for releases prior to Elasticsearch
6.3.0 as a plugin and after that as module.

Relates elastic#60

Embedded elasticsearch is not recommended for production.
Gem version bump

Fixes elastic#60