Added a git::clone define

This additional define duplicates some of the functionalities of the current git::reposync, but is more explicit about the fact that it only clones a repository if if does not exist but provides no mechanism to keep it in sync. It also integrates with the main "git" class by adding a "clones" parameter. This parameter is a hash describing all repositories to be cloned. This allows to only associate the git main class and configure everything else via hiera.

Note that I added a dependency on stdlib to do input parameter validation. This validation is very minimal, so it might make sense to remove it and remove the dependency.

My personal use case is to clone all the projects on which I am working in my home directory whenever I reinstall my workstation.