ctdb-tests: Simplify nodes file handling in tool tests

ctdb-tests: Simplify nodes file handling in tool tests

Instead of using an intermediate environment variable for nodes files,
just create "node" or "nodes.<pnn>" in CTDB_BASE.  This makes the
nodes file loading in fake_ctdb slightly repetitive but simplifies the
test scripts a lot.  It also remove several instance of the CTDB_NODES
variable from the code base, so it is no longer found by "git grep".

Use an empty nodes file to indicate that fake_ctdbd should fail to
read it.

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