iscsi: Specify tpgt to impose creation of a new style node record

There are 2 formats for an iSCSI node record.

An old style format where the path is /var/lib/iscsi/nodes/{target}/{portal}
and a new style format where the portal path is a directory containing a
record file for each bounded iface. For example:

/var/lib/iscsi/nodes/{target}/{portal}/eth0
/var/lib/iscsi/nodes/{target}/{portal}/eth1

Explicitly specifying tpgt(target portal group tag) on iSCSI login imposes
creation of the new style node record which enables to access a portal
through multiple ifaces for multipathing.

Change-Id: I11a6a556d31524f4fae4786f5735accaf1cefb34
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25706
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>