crab submit should fail if certificate validity is too short

[belforte]

currently if certificate expires in N (<30) days, crab can only renew myproxy for N days instead of the default 30 and still submits the task. But TaskWorker will not submit if credential from myproxy is not at least 15 days leading to SUBMISSION FAILED errors which are hard to understand for the user.

Would be better that crab submit command failed immediately with a clear message.

This requires that the minimum myproxy duration needed by TW is shared with client, i.e. moved to ServerUtilities.py

@mapellidario FYI