engine: Set default tpgt value to 1

Engine uses wrong iSCSI terminology, portal instead of tpgt (target
portal group tag) which only purpose at this stage impose iSCSI layer to
create node record in the new format supporting multipathing.

Tpgt default value in VDSM is 1 so it should be the same here.

Change-Id: I7ebfc1335b051e02f62b9544d2500416818940d0
Bug-Url: https://bugzilla.redhat.com/1078137 
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>