Specify lld in tgt config backends

Otherwise when running tgt-admin --delete, it might forget
to invoke tgtadmn with "--lld iscsi" in order to delete the low-level
iscsi exports, which can cause crashes or hangs later on.

Closes-Bug: #1311640
Change-Id: I436c67c30565089236e987d84ac2609eedd2d2c7
(cherry picked from commit 872b7ca0fc5ce4911fdaadee464d46be5a38058d)
Merge "Specify lld in tgt config backends" into stable/icehouse