MB-8429: grab memcached logs in osx too

Clearly grabbing memcached logs is not Linux-specific and thanks to
"server guts" we should have right path regardless of OS or customer
setup.

Change-Id: Ie8320b9a872c035b9b8a809b6caf86e6f02e8022
Reviewed-on: http://review.couchbase.org/26841
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>