election: test join to an election-enabled cluster

There was a bug that a new replica at join to a election-enabled
cluster sometimes tried to register on a non-leader node which
couldn't write to _cluster, so the join failed with ER_READONLY
error.

Now in scope of #5613 the algorithm of join-master selection is
changed. A new node looks for writable members of the cluster to
use a join-master. It will not choose a follower if there is a
leader.

Closes #6127