SECURITY ISSUE: JENKINS_HOME.gitignore must ignore credentials.xml

[cowlinator]

JENKINS_HOME.gitignore must ignore credentials.xml, as that file may contain encrypted secrets.

Also, job config should be included.

Reasons for making this change:

I am a Jenkins user. I decided to use this JENKINS_HOME.gitignore as I version controlled my jenkins configuration.

1. Then I discovered that credentials.xml, which contains encrypted passwords, private ssh keys, and other secrets. In general, it is bad practice and a security issue to include any secrets or credentials-- encrypted or not-- in git repositories. (And in this case, they can be decrypted very easily).
2. I also discovered that the job configurations were being ignored. Job config.xml files are important, sometimes containing a lot of intricate configuration, and in fact sometimes contain <script>{code}</script> which literally contains code. Code should be included in version control.

Links to documentation supporting these rule changes:

1. Not documented. Just enter any credential into a jenkins instance, and look at the credentials.xml file. You will see your encrypted credential. The need for this change is self-evident.
2. https://wiki.jenkins-ci.org/display/JENKINS/Administering+Jenkins