Treat gitlabUrl as a format string for gitlab-runner

DJ Mountneyrequested to merge
runner-format-url into master

If the gitlabUrl contains the '%' they will be processed using go's fmt package. https://golang.org/pkg/fmt/

The two args the formater will be passed are the release name, and the namespace.

This is mostly so we can include the runner chart in our other charts as a dependency, by passing it a format string instead of a defined gitlabUrl.

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