add random password generation for jenkins_user

jenkins-cli can't authorise by private key, if password  for jenkins user is not set.

Jenkins throws exception if user password not set. This path 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