Refactoring

Refactoring

- Wrap common methods that use globals in a Class
 `get_ping_status` and `add_to_summary` depended on globals for
  statefulness. The easiest way to refactor them was to convert them
  into methods and the globals they used into instance variables.
- Move `generate_userdata_common` and `generate_userdata_with_ssh` to
  the utils module.
- Make flavor a kwarg in create_instance.
  Add a defaults section in config.yaml with flavor. This value is used
  if flavor is not passed via kwargs.
- Substitute some sleeps with `wait_for_instances_up` which will
  wait until a dhcp lease success message appears in the instance's log.
- Add functions that wait until router and network association is
  completed. These do not replace the sleeps but make the tests more
  deterministic.
- Remove unused ping_timeout from config.yaml

Change-Id: Ia68ebf47aaa5a58c4e20267f1476db626f015a7c
Signed-off-by: Romanos Skiadas <rski@intracom-telecom.com>
Co-Authored-by: George Paraskevopoulos <geopar@intracom-telecom.com>