IPv6 support in test infra and misc changes

1) Fixtures and introspects are modified to support IPv6
2) image is updated and a new keyword flavor is added
   to notify default flavor
3) Fixed multiple bugs in scripts/vm_regression/test_vm.py
4) Ported most of the scripts in vm_regression/test_vm.py to support v6
5) Removed few unused files
6) Traffic pkg is updated to support IPv6
7) added/updated few rand utility functions

ToDo:
1) Multi-stream support in traffic pkg
2) traffic_tests.py has to be changed to support v6 too
3) Port remaining couple of tests for v6 in vm_regression/test_vm.py
4) Neutron scripts has to be ported for v6
5) v6 has to be enabled in CI env

Change-Id: I39caccc9a21f51d5a64162ed83fbdf3694663836
Partial-Bug: #1391832