Improve functional tests launcher pytest support

Improve functional tests launcher pytest support

Using pytest from the python interpreter can generate unwanted behavior.

https://askubuntu.com/questions/1222958/
why-does-ctrl-c-not-kill-python-running-in-terminal

Typically when using Ctrl-C, subprocess are not properly killed.
This was not happening with nosetests before.
Unfortunately it has been superseded by nose2 which is not properly
working with the current testsuite and is not under active development.

- replace "python3 -m pytest" in launch script by pytest-3 or pytest
  when available.

Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>
Change-Id: I871fa0fce48e176427b07809138a0e4ba1e03247