vagrant docker-exec should include the -w flag for working directory

Vagrant 2.1.1
Windows 10
Docker 18.03.1-ce-win65 (17513)

I've setup a vagrantfile with a custom Ubuntu 16.04 image. It sets up nicely, but `vagrant docker-exec` is not executing in the `/vagrant` directory by default. I double checked the code, and sure enough, no `-w` flag is included in the call to `docker exec`. What's more, the command line wrapper rejects the `-w` flag and prints the help text. The default behavior should be to set the `-w` flag to point to `/vagrant` and to allow me to override that on the command line if I want. It could even be configurable in the `Vagrantfile`, but just the flag is enough to start. The code changes will take all of 2 minutes and will enhance the functionality of the docker plugin tremendously by making it MUCH more convenient.

Start by locating the vagrant docker-exec command wrapper and the Docker exec invocation it uses. Check how command-line options are parsed, then verify that the working-directory option defaults to /vagrant and can be overridden; add or run the relevant command tests if present.

Written by the indexing model from the issue text.