Add HTTPS download options to `box update` and `box outdated`

Vagrant::Box.load_metadata did not provide a way to specify the HTTPS
download options that could be specified when downloading boxes
(ca cert, ca path, client cert, insecure). As a result, while it was
possible to add a box whose metadata file needed to be downloaded with one of
those options specified, it was impossible to check for updates. The following
changes have been made to address the situation:

1. Create a DownloadMixins module to provide the --insecure, --cacert, --capth,
   and --cert command line options to all of `vagrant box add`,
   `vagrant box update`, and `vagrant box outdated`.
2. Extend `Vagrant::Box.has_update?` and `Vagrant::Box.load_metadata` to accept
   said download options.
3. Extend `box outdated` and `box update` commands to pass download options
   down.
4. Extend `Vagrant::Builtin::Action::BoxCheckOutdated` to honour download
   options.
5. Options specified on the command line take precedence over options specified
   in the machine configuration, if any.
6. Fix bug in `vagrant box add` where client cert was being passed down using
   the wrong environment key.
7. Unit test coverage in update_test and box_check_outdated_test.

Resolves #4420