Description of problem:
ovirt-hosted-engine-setup is currently failing if, due to a bad multipath configuration on SAN side, the user tries to access an iSCSI portal where iSCSI discovery finds targets but not one of them is associated with the accessed portal.

Example:
2017-08-31 12:59:04 DEBUG otopi.plugins.otopi.dialog.machine dialog.__logString:204 DIALOG:SEND       ### Please specify the iSCSI portal IP address: 
2017-08-31 12:59:12 DEBUG otopi.plugins.otopi.dialog.machine dialog.__logString:204 DIALOG:RECEIVE    192.168.96.205

2017-08-31 12:59:23 DEBUG otopi.plugins.gr_he_setup.storage.blockd blockd._iscsi_discovery:320 {'status': {'message': 'Done', 'code': 0}, 'items': [u'192.168.96.206:3260,2460 iqn.2007-11.***.************:************-01-v04c505ca8ddf8aed.000000a2.891a8f27', u'192.168.96.207:3260,2460 iqn.2007-11.***.************:************-01-v04c505ca8ddf8aed.000000a2.891a8f27', u'192.168.96.206:3260,2460 iqn.2007-11.***.************:************-02-v04c505ca8ddf8aed.000000a3.891a8f27', u'192.168.96.207:3260,2460 iqn.2007-11.***.************:************-02-v04c505ca8ddf8aed.000000a3.891a8f27']}
2017-08-31 12:59:23 DEBUG otopi.plugins.gr_he_setup.storage.blockd blockd._iscsi_discovery:337 found: []

And then the code fails on:
2017-08-31 12:45:23 DEBUG otopi.plugins.gr_he_setup.storage.blockd
blockd._iscsi_discovery:337 found: []
2017-08-31 12:45:23 DEBUG otopi.context context._executeMethod:142
method exception
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/otopi/context.py", line 132,
in _executeMethod
    method['method']()
  File "/usr/share/ovirt-hosted-engine-setup/scripts/../plugins/gr-he-setup/storage/blockd.py",
line 611, in _customization
    default=valid_targets[0]
IndexError: list index out of range

Version-Release number of selected component (if applicable):
4.1.5

How reproducible:
100%

Steps to Reproduce:
1. run hosted-engine-setup ad try to connect to an iSCSI portal that reports just targets associated to other portals
2.
3.

Actual results:
line 611, in _customization
    default=valid_targets[0]
IndexError: list index out of range

Expected results:
It loops till the user enters a valid configuration

Additional info: