ctdb-tests: Use ctdb_get_all_pnns()

ctdb-tests: Use ctdb_get_all_pnns()

The nested, arbitrary use of onnode can cause messages from ssh on
stderr to appear in the node list:

  Getting list of nodes...
  Output of "onnode -pq all ctdb pnn | grep '^[0-9][0-9]*$'":
  Warning: Permanently added '10.0.0.72' (RSA) to the list of known hosts.^M
  Warning: Permanently added '10.0.0.75' (RSA) to the list of known hosts.^M
  Warning: Permanently added '10.0.0.73' (RSA) to the list of known hosts.^M
  Warning: Permanently added '10.0.0.74' (RSA) to the list of known hosts.^M
  1
  0
  3
  2
  Create/wipe test database rec_test.tdb
  Failed to execute "Permanently added '10.0.0.72' (RSA) to the list of known hosts.^M ctdb attach rec_test.tdb" on node(s) "Warning:"
  Invalid <nodespec>

Avoid that by using an existing function that does not nest ssh uses.

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