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.conf.sample

@@ -42,6 +42,12 @@
 # Optional configuration items
 # ============================
 
+# SSH connection information
+# We recommend using the "postgres" user, so it is enough to indicate the hostname.
+# If extra parameters such as port, etc. are needed, they can be
+# specified in the .ssh/config file.
+#ssh_hostname='192.168.204.104'
+
 # Replication settings
 # ---------------------