1. Jenkins can connect to this slave node when I setup 'admin' user with adding the private key.

[12/01/16 09:00:44] [SSH] Opening SSH connection to 192.168.177.157:22.
[12/01/16 09:00:44] [SSH] Authentication successful.
[12/01/16 09:00:44] [SSH] The remote users environment is:
BASH=/bin/bash
----------------------
<===[JENKINS REMOTING CAPACITY]===>channel started
Slave.jar version: 2.62
This is a Unix agent
Evacuated stdout
Agent successfully connected and online

2. To create a Build job, in the Manage Jenkins --> Configure System --> SSH remote hosts, I add in the 'admin' user and hostname. After that I use this 'admin' user and hostname to set up connection to the slave node in the Build Job. In this setup user and hostname section, it doesn't have an option to add in the private key for the 'admin' user. So it shows "Authentication fail" when I run the Build job using "Execute shell script on remote host using ssh".

3. I try to use the SSH Agent Plugin and follow steps in the Jenkins wiki page to install the "Apache Tomcat Native libraries" on the slave node:
https://wiki.jenkins-ci.org/display/JENKINS/SSH+Agent+Plugin

After I install the Apache Tomcat Native libraries on the slave node, it still shows "Authentication fail".

[ssh-agent] Looking for ssh-agent implementation...
[ssh-agent] Java/JNR ssh-agent
[ssh-agent] Registered BouncyCastle on the remote agent
[ssh-agent] Started.
....
[SSH] Exception:Auth fail
com.jcraft.jsch.JSchException: Auth fail
at com.jcraft.jsch.Session.connect(Session.java:452)
at com.jcraft.jsch.Session.connect(Session.java:150)
at org.jvnet.hudson.plugins.SSHSite.createSession(SSHSite.java:141)
at org.jvnet.hudson.plugins.SSHSite.executeCommand(SSHSite.java:151)
at org.jvnet.hudson.plugins.SSHBuilder.perform(SSHBuilder.java:60)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:779)
at hudson.model.Build$BuildExecution.build(Build.java:205)
at hudson.model.Build$BuildExecution.doRun(Build.java:162)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:534)
at hudson.model.Run.execute(Run.java:1720)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:404)
Build step 'Execute shell script on remote host using ssh' marked build as failure

Please give your advice,
Thank you