Allow force removal of instances

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