Refactor tests launching procedure

Refactor tests launching procedure

The possibility to run test separately was lost during preliminary work
to prepare jobs parallelization.
This refactoring restores that feature but with a different approach.
For example, the equivalents of the old commands
    $ tox -e topoPortmapping
and
    $ tox -e portmapping,pce
are now
    $ tox -e tests121 topo_portmapping
and
    $ tox -e tests121 "portmapping pce"
(preceded by $ tox -e buildcontroller)

The possibility to choose the script launcher command from a global
variable have been added.
For example,
    $ export LAUNCHER='python3'
allows to replace 'nosetests --with-xunit' by a straight call to python3
to have more debug output during tests.

Tests order can also be changed by directly modifying script filenames
without touching tox.ini. Filenames have been modified here to add a
numbering and keep the tests current order.

JIRA: TRNSPRTPCE-482
Signed-off-by: Guillaume Lambert <guillaume.lambert@orange.com>
Change-Id: I725d1dfac42bd309d18fbf87ff1c85bc1bce5be9