Fix chroot builder to work with chef-solo provisioner

According to be5adb92b5c4d5d95e5089be94c706b930fea7ba, the `UploadDir` method supports two ways of copying depending on whether a trailing slash is used:

    src = "dir"  -> dest/dir
    src = "dir/" -> dest

On BSD-based systems (such as OSX, FreeBSD, etc.) the `cp -R` command handles these two cases automatically.  However, Linux treats "src/" and "src" the same.

To support the trailing slash syntax portably, we can use:

    src = "dir"   -> dest/dir
    src = "dir/." -> dest

This works on BSD and Linux.  It is better than using wildcards as it grabs hidden files as well.

This fixes #1196 (chef-solo provisioner doesn't work with chroot builder).