Networking: Move multicast discovery to a plugin

Multicast has known issues (see #12999 and #12993). This change moves
multicast into a plugin, and deprecates it in the docs.  It also allows
for plugging in multiple zen ping implementations.

closes #13019

dev-tools/create_bwc_index.py

@@ -141,7 +141,6 @@ def start_node(version, release_dir, data_dir, repo_dir, tcp_port=DEFAULT_TRANSP
     '-Des.path.logs=logs',
     '-Des.cluster.name=%s' % cluster_name,
     '-Des.network.host=localhost',
-    '-Des.discovery.zen.ping.multicast.enabled=false',
     '-Des.transport.tcp.port=%s' % tcp_port,
     '-Des.http.port=%s' % http_port,
     '-Des.path.repo=%s' % repo_dir