Tribe Node
The tribes feature allowed to create a tribe node that can act as a federated client across multiple clusters.

The tribe node configuration looks something like this:

```
tribe.t1.cluster.name: cluster1
tribe.t2.cluster.name: cluster2
```

The configuration above configure connections to 2 clusters, named `t1`, `t2`. It creates a "node" client to each (so by default, above, multicast discovery is used). The settings for each node client is extracted from the `tribe.[tribe_name]` prefix.

The way the tribe node works is by merging the cluster state from each cluster, and creating a merged view of all clusters. This means all operations work the same, distributed search, suggest, percolation, indexing, ... .

The merged view drops conflicted indices and picks one of them if there are 2 indices with the same name across multiple clusters.

By default, read and write operations are allowed. Master level read operations (cluster state for example), require setting the local flag to true (since there is no elected master). Master level write operations are not allowed (create index, ...).

The tribe node can be configured to block write operations `tribe.blocks.write` to `true`, and metadata operations by setting `tribe.blocks.metadata` to `true`.
closes #4708