Refactor some common methods into utils

Refactor some common methods into utils

- generete_ping_userdata(): This method was exactly the same in two
  tests
- create_instance(): The various implementations of this method had
  almost the same signature, but not quite. This was fixed by adding
  extra arguments as needed. Also a global variable was used in the
  function, which was converted into a parameter.
- create_network(): The three implementations of create_network had
  three different signatures and returned different variables. They
  were all unified in a single create_network class and the calls to
  create_network were adjusted accordingly.

Revision 1: Fix flake8 violations
Revision 2: Rename logger in utils

Change-Id: Ib6cfe798dc561a69eb50c901e3aa71c19d465821
Signed-off-by: Romanos Skiadas <rski@intracom-telecom.com>