This change moves providing UnicastHostsProvider for zen discovery to be
pull based, adding a getter in DiscoveryPlugin. A new setting is added,
discovery.zen.hosts_provider, to separate the discovery type from the
hosts provider for zen when it is selected. Unfortunately existing
plugins added ZenDiscovery with their own name in order to just provide
a hosts provider, so there are already many users setting the hosts
provider through discovery.type. This change also includes backcompat,
falling back to discovery.type when discovery.zen.hosts_provider is not
set.
60353a245a