ctdb-recoverd: Add PNN to recovery daemon context

ctdb-recoverd: Add PNN to recovery daemon context

This is currently referenced in a number of inconsistent
ways, including:

* pnn
* rec->ctdb->pnn
* ctdb->pnn
* ctdb_get_pnn(ctdb)
* ctdb_get_pnn(rec->ctdb)

The first of these always requires some thought about the context - is
this the node PNN or some other PNN (e.g. argument to function)?

The intention is to always use rec->pnn when referring to the recovery
daemon's PNN.

Doing this also reduces reliance on struct ctdb_context internals.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>