LU-569: Make lu_object cache size adjustable

lu_object cache is specified to consume 20% of total memory. This
limits 200 clients at most can be mounted on one node. We should make
it adjustable so that customers have a chance to configure it by
their needs.

Change-Id: I8dcb993c88a3abd9fc8ba11ff1578aa3897c3933
Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1188
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>