Only search gitlab:import:repos on repos on subdir

of type `repos_path/group/repo.git`.

Before this, any other location like `repos_path/repo.git`
or `repos_path/group/XXX/repo.git` does generate new projects, but
these projects are empty since no copy is done.

Now `group_name == '.'` since glob does not expand to `.`, so that logic was removed.

Fix https://github.com/gitlabhq/gitlabhq/issues/4137
IIUC `.save` already does validations and returns false on fail: the difference is that `save!` would raise: http://guides.rubyonrails.org/active_record_validations.html#when-does-validation-happen-questionmark