New feature: Improve BYOJ (Bring Your Own Jenkins) scenario

[bleggett]

Split out from #110

We assume you are running a Jenkins server on a RHEL/CentOS instance. This is a little shortsighted, and we should try to realistically support every platform the Chef provisioner understands.

Now that the Chef cookbooks have been tidied it should be easier to get them working on e.g. Ubuntu or Debian.

Note that "improve" might also mean documenting how to manually prep your Jenkins server for use by Jazz, with a list of the prerequisites we expect to be installed on the box, for people that would rather install the prerequisites manually rather than running our Chef cookbook against their server.