Update stage-build to run tox sequentially

Change-Id: I967f19a8c35700290e93c9d8bd863b63b7c2d239
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
(cherry picked from commit ea063c7a6ae6a5d7e11e8c22f9707d5c8f674ac7)
2 files changed