Add slashes to mongodb's managed_files.yml

Add trailing slashes to directories in managed_files.yml of the mongodb
cartridge.

Although the absence of the slashes was not causing any apparent
problems, correctness and consistency with other cartridges demands that
directories be denoted with trailing slashes.
Merge pull request #3053 from Miciah/cartridges-mongodb-metadata-managed_files.yml-add-trailing-slashes

Merged by openshift-bot