Medium: iscsi: use have_binary, not check_binary, in open_iscsi_setup

open_iscsi_setup used check_binary which immediately exits with
OCF_ERR_INSTALLED when required binaries are not installed. This
breaks probes, which should exit with OCF_NOT_RUNNING in this
case. Use have_binary and _return_ OCF_ERR_INSTALLED from the function
instead.