tests: Fixed load balancing system-tests

Most load balancing system tests were randomly failing from time to time
as they were checking that, after 20 requests sent to load balancer, all
backends were at least reached once.
Statistically, this was failing from time to time.
Now, if after 10 requests we did not get the expected distribution, we
send 10 more requests. We do that around 30 times

Signed-off-by: Xavier Simonart <xsimonar@redhat.com>
Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
(cherry picked from commit 1791a10)