tests/functional: add a new test for iSCSI multipath

The new test checks whether the iSCSI multipath functionality of
sheepdog works collect or not. I excluded it from auto group because
it heavily depends on distribution specific stuff (e.g. configuration
file of multipathd and open-iscsi).

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>