Discover all clusters using auto discovery

[alex-zaitsev]

Currently, cluster auto discovery requires to specify a cluster name, and then it discovers cluster nodes. It is ok if we want to cluster to know about its own nodes, but does not work well when we want to discover remote clusters.

Instead, we should allow to discover all clusters under a certain path on observer node.

Current docs https://clickhouse.com/docs/en/operations/cluster-discovery#using-cluster-discovery

Proposed syntax:

<remote_servers>
    <discovery>
        <path>/clickhouse/discovery</path>
        <observer>true</observer>
   </discovery>
</remote_servers>

That should work together with existing clusters that are defined explicitly

[ianton-ru]

#629

[ianton-ru]

Little bit different syntax to avoid confusing with clusters (if someone already has cluster with name discovery

<remote_servers>
  <any_fake_unused_name>
     <discovery>
       <multicluster_root_path>/clickhouse/discovery</multicluster_root_path>
       <observer>true</observer>
    </discovery>
  </any_fake_unused_name>
<.remote_servers>

[alex-zaitsev]

Closed by #629