Add repo retention count

[mattladany]

This new configuration value adds the ability to not delete <repoRetentionCount> number of images from each docker repository within the Artifactory repository. Without this, if all images have not been downloaded or modified within the retention specified, all images will be removed for that docker repository.

My use case (though I'm sure there are others) is that I'm using a specific image as a base for building other images. However, every developer has already downloaded the image months in the past, and all production nodes have also downloaded the images months in the past. If a production node goes down and a new one needs to be spun up, or a new developer joins the team, this image still needs to exist within Artifactory and should therefore not be deleted. This repoRetentionCount configuration value provides the ability for that image to never get deleted until it is updated or replaced by a newer version.

[crazy-max]

I think you should consider setting Docker tags you wish to retain as semver and use the keepSemver option instead. You can also use the exclude option to exclude tags matching a regular expression.

Your proposal still LGTM though but I think we should name it retentionCount.