add random password generation for jenkins_user

`jenkins-cli` cannot authorize by private key, if the password for the `jenkins` user is not set.

Jenkins throws exception if user password not set.

This patch generates secure random password for each new registered user by `jenkins_user` helper. 

https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/security/HudsonPrivateSecurityRealm.java#L167