* self is initially not member (in gossip state)
* if the join to seed nodes timeout it joins itself, and becomes
singleton cluster
* remove the special case handling of singelton cluster in gossip
merge, since singleton cluster is not the normal state when joining
any more
* Implement the join to seed nodes process
When a new node is started started it sends a message to all
seed nodes and then sends join command to the one that answers
first.
* Configuration of seed-nodes and auto-join
* New JoinSeedNodeSpec that verifies the auto join to seed nodes
* In tests seed nodes are configured by overriding seedNodes
function, since addresses are not known before start
* Deputy nodes are the live members of the seed nodes (not sure if
that will be the final solution, see ticket 2252
* Updated cluster.rst with latest info about deputy and seed nodes
