title: cannot connect Jenkins slave with master
body: <p>I have installed a Jenkins master on my Mac OS, version is 2.90.<br>
  I have installed a Jenkins slave on my ubuntu, and created a user called jenkins at /home/jenkins.<br>
  I have copied my pub key from master to slave and using</p>
  
  <pre><code>ssh jenkins@slave_ip
  </code></pre>
  
  <p>I can login from master to slave with no password.<br>
  However, I cannot create a new slave at Jenkins, it has the following issues:<br>
  1. Launch method does not have option "Launch slave agents via SSH",therefore I cannot set credentials.<br>
  2. since launch method only has "Launch agent via execution of command on the master" and another option for windows, I input Launch command as: ssh -v jenkins@slave_ip. 
  but master cannot connect with slave. </p>
  
  <p>To troubleshoot, I login slave manually, I see master does ssh to slave successfully. as it shows:  </p>
  
  <pre><code>sudo tail -f /var/log/auth.log:
  Nov 19 00:10:32 ip-172-31-18-180 sshd[8908]: Accepted publickey for     
  jenkins from my_master_ip port 62411 ssh2: RSA     
  80:23:0e:1b:34:c1:90:52:a6:df:d0:24:6f:10:80:73
  Nov 19 00:10:32 ip-172-31-18-180 sshd[8908]: pam_unix(sshd:session):    
  session opened for user jenkins by (uid=0)
  </code></pre>
  
  <p>so I doubt maybe Jenkins credential is not set because master does not send private key to slave. (I do not have this option in my Jenkins UI)</p>
  
  <p>what should I do? how to make Launch method option "Launch slave agents via SSH" show, so that I can input credential of master?</p>