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.

bug url: https://bugzilla.redhat.com/show_bug.cgi?id=1024263
Change-Id: Ia7806316f7cdfaa7d6f42323e6db45886a97f9d6
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>