Scenario : Start instances using fixed network when possible

Uses the fixed_network_name option to select which network
to boot test instances on.  This allows Tempest to be run
in environments where there are multiple networks available.

Change-Id: I64ad6ce6bee3fe33537ac4f8d8d913856fe46554
Partial-Bug: #1250866
Merge "Scenario : Start instances using fixed network when possible"