MB-100 Use executing_on_new_process in wait_for_nodes.

It handles corner cases better than the code that was in place
there. For instance, old code would stuck forever if child process
died.

Change-Id: Ia8292d7266507e71e21622c4f6092c11bb84776b
Reviewed-on: http://review.couchbase.org/28868
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>