Store configuration file in repmgr.nodes table

When executing repmgr on remote nodes, we otherwise end up jumping
through hoops as we can't make assumptions about where the configuration
file is located, but really need to be able to provide it.

From a support point of view it will also make life easier as it will
be easy to specify exactly which file to provide.

repmgr--4.0.sql

@@ -11,7 +11,8 @@ CREATE TABLE nodes (
   priority         INT         NOT NULL DEFAULT 100,
   conninfo         TEXT        NOT NULL,
   repluser         VARCHAR(63) NOT NULL,
-  slot_name        TEXT        NULL
+  slot_name        TEXT        NULL,
+  config_file      TEXT        NOT NULL
 );
 
 CREATE TABLE events (