cluster matrix: make remote execution of cluster show more configurable

- use the remote user setting, like other SSH-based remote operations
  (avoid hardcoding the user name)
- enable `repmgr cluster show` to accept the cluster name and the
  database connection information instead of requiring repmgr.conf;
  this means we don't have to assume that repmgr.conf is in one
  of the default locations

repmgr.h

@@ -61,6 +61,7 @@
 #define OPT_WITHOUT_BARMAN               10
 #define OPT_NO_UPSTREAM_CONNECTION       11
 #define OPT_REGISTER_WAIT                12
+#define OPT_CLUSTER                      13
 
 /* deprecated command line options */
 #define OPT_INITDB_NO_PWPROMPT           999