ctdbd: Add nodes_file member to struct ctdb_context

ctdbd: Add nodes_file member to struct ctdb_context

This allows ctdb_load_nodes_file() to move to ctdb_server.c and
ctdb_set_nlist() to become static.

Setting ctdb->nodes_file needs to be done early, before the nodes file
is loaded.  It is now set from CTDB_BASE instead ETCDIR, so setting
CTDB_BASE also needs to be done earlier.

Unhack ctdbd_test.c - it no longer needs to define
ctdb_load_nodes_file().

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

(This used to be ctdb commit 20e705e63bd3b20837cc3ac92fdcf2a9650ccfc8)