support auto-detection of the updatebot.yml file

[jstrachan]

it'd be nice if we had a simple convention over configuration way to find the updatebot.yml so we don't have to mention it explicitly inside the Jenkinsfile

e.g. look in this order:

• look on the CLI / system property / env var
• look in the current directory
• if its not there and the current project is a github repository, find the github org. Then for the github org foo look for the file in github at https://github.com/foo/updatebot-config/.

We may also want to chain together github organisations.

e.g. the configuration in fabric8io/updatebot-config could point to other organisations to look in for the next round of PRs to generate - e.g. to point fabric8io at fabric8-services, fabric8-ui, fabric8-quickstart etc.

Then each organisation is modular and we can chain github organisations together easily