Improve default ssh configuration of computers

[giovannipizzi]

Currently, during computer configure, the look_for_keys is set to False by default. Better to set it to True (paramiko's default). Also, I would set allow_agent to True (on the shell, if you have a SSH agent, you expect that it is automatically used by ssh).
Finally, if no key is specified (empty string), this should just not be passed to paramiko. Currently, during a verdi computer test, I get an error

[SshTransport] [Errno 2] No such file or directory: '', connect_args were: {
'gss_host': 'daint.cscs.ch', 
'port': 22, 
'look_for_keys': True, 
'timeout': 60, 
'gss_deleg_creds': False, 
'compress': True, 
'username': 'pizzi', 
'sock': <aiida.transports.util._DetachedProxyCommand object at 0x7f0251adaa20>, 
'gss_auth': False, 
'key_filename': '', 
'allow_agent': True, 
'gss_kex': False
}