Currently, an unreachable node (or one that return undetermined failure)
in the hooks pre-phase will abort the curren operation. This is not
good, as a down node could prevent many operation on the cluster.

This patch changes a RPC-level failure (and not a hook execution
failure) into a warning. It also modifies the related test cases.

This fixes issue 11.

Reviewed-by: ultrotter

Reviewed-by: schreiberal

This patch adds a new option to the instance removal command
"--ignore-failures" that forces the removal of the instance from the
configuration even if the removal process encounters errors.

In order to be able to do this when the remote node(s) is(are) down, we
need to restrict the execution of the instance removal hook to the
master only. I think this is a reasonable trade-off (but I'm not sure).

Reviewed-by: imsnah

- Add Makefile in doc/examples/

Reviewed-by: schreiberal