Improve test_load_balancer_basic

This patch introduces multiple improvements of test_load_balancer_basic.

 1. Use inetd instead of nc on backends.
    Running nc in a while True cycle proved to be ineffective
    and unreliable in case of a relatively big number of
    requests.
 2. Add doc string to NeutronRetriever.
 3. Add setUp method and cleanups for resources
    as a preparation for adding more tests.
 4. Other minor fixes of storing internal values.

Change-Id: I8b5426c9cc832cb8bb5024952535f312cd627d1e
Merge " Improve test_load_balancer_basic"