Fix warnings in all snaps-related modules

Fix warnings in all snaps-related modules

It also adds vping/__init__.py to stop skipping vping unit tests.
It removes the useless import os which is not required for mock (here
the mock patch was simply false).

It should be noted that the code is partially duplicated between
snaps_test_runner.py and vping_base_base.py. A small part of code has
been rewritten to fix that without multi inheritance.

Change-Id: I2e3e181ad7dab8aa41afb1e63d0b6795ff0d1610
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>