Discovery: Support local (JVM level) discovery

Allow to have a JVM (well, actually class loader) level discovery for simple testing / embedding of a single node (which, potentially exists with other nodes in the same class loader).

Enable it using:

```
discovery:
    type: local
```

Or using:

```
node:
    local: true
```

(which will also enable other modules to be local, such as the transport - once we have that...)
