Fix MismatchError for LB scenario test

It has been observed that at a given iteration only one of the two HTTP
services responds to requests made to the VIP. This is somewhat rare and
it may be caused by the fact that the number of requests being sent is
not high enough for the ROUNDROBIN algorithm to equally distribute requests
across all the members in the pool.

Increasing the number of retries makes the test runtime only slightly
longer, but it should give us more confidence that, if we keep observing
the same type of failure, this can be ruled out.

At the same time, this patch also relaxes the check so that it just verifies
that member pools are load balanced at least once. Verifying that the number
of requests are spread evenly across is always going to be prone to transient
condition and it is beyond the scope of a scenario test that validates whether
Neutron LBaaS is wiring things correctly.

Closes-bug: #1358857

Change-Id: Ibc0aeb092a4643eb151cef8d7d942d9e3d1ed71c
Merge "Fix MismatchError for LB scenario test"