Make ctdb_diagnostics more resilient to uncontactable nodes.

Make ctdb_diagnostics more resilient to uncontactable nodes.

Current behaviour is for onnode to timeout (for about 20s) for each
attempted ssh to a down node.  With 40 or 50 invocations of onnode
this takes a long time.

2 changes to work around this:

* If EXTRA_SSH_OPTS (which is passed to ssh by onnode) does not
  contains a ConnectTimeout= setting then add a setting for a 5 second
  timeout.

* Filter the nodes before starting any diagnosis, taking out any "bad
  nodes" that are uncontactable via onnode.

  In the nodes summary at the beginning of the output, print
  information about any "bad nodes".

Signed-off-by: Martin Schwenke <martin@meltin.net>