Make Test&Push resilient to addition of new addons

Steps to reproduce:

• launch a test&push job
• while the tests are beeing run, add a new addon on the parent branch
• even if tests went ok, the test&push fails when creating the PR or pushing changes because of the new addon addition.

This happened on https://qa.nuxeo.org/jenkins/job/TestAndPush/job/ondemand-testandpush-atchertchian-master-3/1/consoleText, the nuxeo-platform-lang-ext-incomplete addon was merged by another test&push job while this one was running:

+ for sub_dir in '$(_retrieve_modules)'
+ cd nuxeo-platform-lang-ext-incomplete
/opt/jenkins/workspace/TestAndPush/ondemand-testandpush-atchertchian-master-3/scripts/nxutils.sh: line 49: cd: nuxeo-platform-lang-ext-incomplete: No such file or directory
Build step 'Execute shell' marked build as failure

Attached complete console output.