[XTESTING] Fix opnfv/xtesting version to wallaby

[XTESTING] Fix opnfv/xtesting version to wallaby

opnfv/xtesting master triggered a change in the alpine baseline image (3.14)
As a consequence the xtesting docker relying on master were broken, as the
list of testcases was not overwrittent in pytho3.9 lib folder

2 options were possible:
- fix the version (wallaby)
- align the xtesting dockers and change the path to use 3.9

option1 is more secured.
For the moment python3.8 is enough for the SDK.
If needed we could upgrade later.

Issue-ID: TEST-359

Signed-off-by: morganrol <morgan.richomme@orange.com>
Change-Id: Iaeada26416e0929a11c17f5c2803448c5a23888c
(cherry picked from commit 2e7b7aa9773c4aa8e0334a1398bb12171e6b53d8)