Add option --namespace to command request-branch
    
    This patch adds option --namespace to command request-branch, which
    could be used with --repo together. Previously, packagers could request
    a branch with global option --module-name in either form name or
    namespace/name. --module-name has been replaced with options --name and
    --namespace which should also be usable for request-branch as well.
    
    As a result, packagers could request a branch with either global
    options:
    
      fedpkg --name name [--namespace namespace] request-branch branch
    
    or command level options
    
      fedpkg request-branch --repo name [--namespace namespace] branch
    
    The first form is for backwards compatibility in a way and the second
    form is the recommended way to use in practice which would be more
    straightforward.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>