MB-100: wait long enough for child processes shutdown

We spotted that we waited for memcached only for 1E6 milliseconds
which is less than we thought. We found it was caused by
supervisor_cushion timeout which makes sense to bump to infinity.

Change-Id: Ifc4751ab0b4e44648918a5c291be6299d3cf95db
Reviewed-on: http://review.couchbase.org/26651
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>