Add functions to generate update center certificates

[oleg-nenashev]

In order to operate correctly, Jenkins update sites need to specify private/public keys and X509 certificates. These certificates should be placed into the root of Update site.

It would be great if Juseppe could generate at least self-signed certificates for such purpose.

[lanwen]

Think openssl shell command is enough, isn't it?
Or u can simply disable sign check for bundled update site. What for it needed in this case?

[oleg-nenashev]

1. Signature checks cannot be easily disabled in the current core version
2. But it's quire easy to programmatically add a TrustAnchor
3. A valid certificate + TrustAnchor based on the filepath make the update site to work transparently

So I think it may be a useful addition to the library. Generation of these files by default - TBD

[lanwen]

1. add ability to ignore sign check for update site update-sites-manager-plugin#2
2. Custom sign validator: https://github.com/jenkinsci/update-sites-manager-plugin/blob/master/src/main/java/jp/ikedam/jenkins/plugins/updatesitesmanager/internal/ExtendedCertJsonSignValidator.java

its already done.