Fixes for a non-Vagrant/vbox environment

Changes I made in the course of trying to use an EC2 instance in place of a launcher VM:
- Normalize container names as we get them from Docker, to prevent us from seeing leading slashes all over the place.  This was, for some reason, causing problems when `DOCKER_HOST=unix://...`, but not when `DOCKER_HOST=tcp://...`.
- Loosen the dependency on `vboxmanage`.  Don't enforce that it's present if we're never going to call it.

`pickett inject` still depends on `vagrant ssh`, which we'll want to fix at some point if we want to support non-Vagrant environments.