Add "cluster matrix" mode and "ssh_hostname" parameter

- The "cluster matrix" command supports CSV mode via the --csv
  switch.
- Add the optional ssh_hostname configuration parameter, which is
  required by "cluster matrix".
- A corresponding ssh_hostname column has been added to the repl_nodes
  table and to the repl_show_nodes view.

repmgr.sql

@@ -17,6 +17,7 @@ CREATE TABLE repl_nodes (
   cluster   	text    not null,       -- Name to identify the cluster
   name			text	not null,
   conninfo      text    not null,
+  ssh_command   text,
   priority  	integer not null,
   witness   	boolean not null default false
 );
@@ -65,6 +66,6 @@ CREATE INDEX idx_repl_status_sort ON repl_monitor(last_monitor_time, standby_nod
  * in each case (when appliable)
  */
 CREATE VIEW repl_show_nodes AS 
-SELECT rn.id, rn.conninfo, rn.type, rn.name, rn.cluster,
+SELECT rn.id, rn.conninfo, rn.ssh_command, rn.type, rn.name, rn.cluster,
 	rn.priority, rn.active, sq.name AS upstream_node_name
 FROM repl_nodes as rn LEFT JOIN repl_nodes AS sq ON sq.id=rn.upstream_node_id;