Add a feature to ignore existing git repo on disk
    
    In the traditional use of pagure, if a repo exists on disk we want to not
    create the project, but when using pagure as a front-end for a system
    that handles creating the project on disk via a different process, in that
    situation we want to be able to create the project in the DB while ignoring
    the fact that there is/are already git repos on disk.
    
    The flag ``ignore_existing_repo`` added to pagure.lib.new_project() will
    allow that.
    
    Add corresponding tests