Adapter: Return latest Chart version in container if not specified

If Chart version is not supplied in metadata.labels.chart, then return the latest version of the Chart available in the Operator image under /charts.

Currently, if the version is not provided, it simply returns an empty string, and the Operator will fail to find /charts/gitlab-.tgz.

The Makefile currently handles setting this label on the CR, but end users may not use that method of installation. We should consider supporting kubectl apply -f mygitlab.yaml without script-y injections.

Note: ideally, I think it makes sense to put the Chart version in the CR Spec rather than in metadata.labels, but that can be a follow-up topic.

Closes #87 (closed)