Separate ssh connection logic from constructor

Implementation class of SshConnection has connection logic in
constructor. So instance might not be generated if unstable connection.

This patch separates connection logic from constructor. Then it is moved
to new method connect().
Add byte array for private key phrase
Add interface to update authentication just before ssh session opened.

This is a part of JENKINS-21637.
Fix test and checkstyle errors
Merge pull request #12 from rinrinne/credential-plugin

Add interface to update authentication just before ssh session opened