Update configuration to allow multiple settings per data directory

As discussed in gitlab-ce#28447, we'll need a mechanism to specify a Gitaly address for each repository storage. This will imply a change in the structure of the storage settings. We are probably not going to be able to get the required Gitaly changes into 9.0, but updating the structure of the settings now to allow for extra values per storage will prevent us from introducing breaking changes in minor versions

See also gitlab-ce!9597