get sum of RSS of all erlang processes instead of max RSS per sample

Change-Id: I66d7fdccedd9659cad6e82d8ff0d89feabb861d4
Reviewed-on: http://review.couchbase.org/42720
Reviewed-by: Venu Uppalapati <venumahesh@gmail.com>
Tested-by: Venu Uppalapati <venumahesh@gmail.com>