Add _interface attribut in BaseNetworkTest

This avoid the situation when a child class uses the _interface
attribut but it's not defined.

Other Base class (Volume, Image) seems to do a similar thing.

Patch suggested by Kirill Shileev on LP
Closes-Bug: 1360290

Change-Id: I1cdb52ab87134390cfa5f644cded38e83bf1e77c
Merge "Add _interface attribut in BaseNetworkTest"