Hoststack perf infrastructure refactoring

- DUT only topology (hoststack test
  apps are co-located with vpp)
- Make vpp app specific keywords generic
  where applicable
- Add IP4 Prefix to topology file
- Support running wrk in linux namespace
- Refactor namespace cleanup
- Remove redundant namespace creation code
- Refactor test/keyword dirs: tcp -> hoststack
- Add hoststack utility keywords
- Refactor wrk suite setup/teardown
- Update tests with recent perf infra changes

Change-Id: Ia1cf07978d579393eef94923819a87c8c1f36f34
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>