gluster: correct parameter for creating hookpath in verb hookAdd.

parameter hookLevel must be in lower case for creating correct
path for hook. so that parameter is changed to lower case.

Change-Id: Ic16885a7f837ccceede2cf1b70b5da3138a12a60
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1028972
Signed-off-by: ndarshan <dnarayan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21017
Reviewed-by: Aravinda VK <avishwan@redhat.com>
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21538
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Tested-by: Yaniv Bronhaim <ybronhei@redhat.com>