Zen2 nodes will bootstrap themselves once they believe there to be no remaining
Zen1 master-eligible nodes in the cluster, as long as minimum_master_nodes is
satisfied.
Today the bootstrap configuration comprises just the ids of the known
master-eligible nodes, and this might be too small to be safe. For instance, if
there are 5 master-eligible nodes (so that minimum_master_nodes is 3) then the
bootstrap configuration could comprise just 3 nodes, of which 2 form a quorum,
and this does not intersect other quorums that might arise, leading to a
split-brain.
This commit fixes this by expanding the bootstrap configuration so that its
quorums satisfy minimum_master_nodes, by adding some of the IDs of the other
master-eligible nodes in the last-published cluster state.
eadcb5f0f8