Increasing the amount of open file descriptor for vdsm user

As part of fixing fds leak we increase the amount of available open fds
to allow stability in large scale of simultaneously operations.
Currently lv operations can't run simultaneously and
wait for pior operation to end. Simultaneously attach sd requests open
fds and close them only after end, when operation waits on lv operation
vdsm keeps the fds open for long time.

Change-Id: Ide73b075c56c83d0298a9cd3a0892b0ce7cd46f6
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=920532
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13566
Reviewed-by: Dan Kenigsberg <danken@redhat.com>