Update getRandomNode() to work with credentials:

  - Providing credientials in the form:

 ```php
      new Cassandra\Database([
        '192.168.0.2:8882' => [
          'username' => 'admin',
          'password' => 'pass',
        ], 'my_keyspace');
```

    as the documentation suggests previously failed due to the use of
    `array_shuffle` which squashes non-numerical keys into int indexes.
  - This commit fixes the issue by using `array_rand` which preserves the
    keys.