Restore the persistent server option in the LB tests
Restore the persistent server option in the LB tests

The cb85dcdf5e2cf53ac21bb97382e03a8c5b47042f putted the
nc command into loop in order to tolerate busybox/nc versions
without persistent server option (cirros 0.3.3).

Unfortunately the above change also changed -ll option to -l.
The nc with -ll option provides 'never stop listening' service,
without this behavior the test can fail frequently/always
in several setup.

This change restores the -ll option, which does not causes issues
when you use  cirros 0.3.3 image, and gives stability with cirros 0.3.2
 + haproxy 1.5.x.

Note: The gate is using 0.3.2 cirros and HaProxy 1.4.x,
On the gate this change effectively restores the behavior before
cb85dcdf5e2cf53ac21bb97382e03a8c5b47042f.

Change-Id: I87bee99946447dee499ab3b8ca2b1cacb54668fd
Related-Bug: #1408922
Related-Bug: #1384708

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