tests: add iscsi storage functional test

Create iSCSO storage backend and create vdsm storage layout on it.
Use LIO as iSCSI target and skip the test if LIO is not available.

Change-Id: Icaef5d6145b8f493b5eaab0b75564f88a4cb82ce
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/9842
Reviewed-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>