make sure ssh do not ask password

For some reason, ssh seems to ask for password even when
PasswordAuthentication is set to no, adding PreferredAuthentications
with the 2 options removed do the trick.


To verify that, just take a server where you do not have ssh key: 

ssh -v  -o 'KbdInteractiveAuthentication=no' '-o' 'PasswordAuthentication=no' '-o' 'PreferredAuthentications=hostbased,publickey'  test.example.org 

=> no password asked

ssh -v  -o 'KbdInteractiveAuthentication=no' '-o' 'PasswordAuthentication=no'   test.example.org 
=> ask password

Tested from a F19 and RHEL 6 to a Centos 6 server.