Fixing casual failing of nc service in LBaaS

Netcat quits after first web connection to it, causing test
to fail with HTTP 503 error. In Cirros 0.3.3 nc doesn't support
"-ll" option for persistence listening.
Added catching HTTP exceptions, and infinite cycle for netcat.
Fixed network finding. Fixed command string to be more pythonic.
Closes-Bug: #1384708

Change-Id: Ib9c9cd3d0040003d0296426d8a7ce800c193f22b
Merge "Fixing casual failing of nc service in LBaaS"