Store the replication user in repmgr.nodes

When creating recovery.conf outside of "repmgr standby clone",
there was no way of knowing if a replication user had been
explicitly provided with --replication-user, meaning the value
of "primary_conninfo" would be set to the "conninfo" field of the
node's upstream node record.

We'll add an extra column to store the replication user for each
node so it can be referenced at any time.

repmgr--4.0.sql

@@ -10,6 +10,7 @@ CREATE TABLE nodes (
 
   priority         INT     NOT NULL DEFAULT 100,
   conninfo         TEXT    NOT NULL,
+  repluser         TEXT    NOT NULL,
   slot_name        TEXT    NULL
 );