Only allow public key auth in test-ce sshd, adjust pam config

@gillardo @natthan.pigoux

Could you maybe test if disabling the password authentication fixes having to disable apparmor?

I don't have easy access to a system with apparmor to test.

To test, either run the tests or just something like:

kubectl exec -it deployment/wms-site-director -- ssh dirac@dirac-ce -i .ssh/diracuser_sshkey echo "It worked"

Closes #106 (closed)