KeepLatestNFilesInFolder not working properly when more than one repository is selected

Hi all,

When filtering by ```RepoByMask``` instead of ```Repo```, multiple repositories could be selected.

In this case, the rule ```KeepLatestNFilesInFolder``` won't work properly because doesn't use the repository name for grouping the artifacts: https://github.com/devopshq/artifactory-cleanup/blob/master/artifactory_cleanup/rules/keep.py#L134

My proposal is to modify it to something like this:
```
for artifact in result_artifact:
path = '{repo}/{path}'.format(**artifact)
artifacts_by_path[path].append(artifact)
```

It would be nice to add the repo name to this print too: https://github.com/devopshq/artifactory-cleanup/blob/master/artifactory_cleanup/rules/keep.py#L146

Thanks for this awesome tool!