Retry the restart of nscd up to 3 times

nscd tends to crash, fail to restart... and all this is random.  We try
three times before giving up.
Add test for NSCD restart
Invalidate the NSCD cache inside the loop too

It may fail even if the daemon has started successfully
Fix typo in an SSSD test

It was making the test fail at random.
Merge pull request #179 from piojo/authconfig-nikhef-fix

Retry the restart of nscd up to 3 times