Migrate test_load_balancer_basic to tempest client

This patch migrates the load balancer basic ops scenario test from official
client to tempest clients.

It also removes the NeutronRetriever class from the test script

Partially-implements: bp:tempest-client-scenarios

Change-Id: I4f3da202646821671d0e98707031a49973c256b2
Merge "Migrate test_load_balancer_basic to tempest client"