Allow container name to be set explicitly or to be set to the vagrant machine_name

Another PR to fix fgrehm/vagrant-lxc#132
This is based on ccopes'  https://github.com/fgrehm/vagrant-lxc/pull/199, but includes the ability to set the container name explicitly as well as setting it to match the machine name. It also includes an update to the readme. I'd love to see either this or ccopes PR  merged sometime soon, as having no control over container is a big deal (at least here). 

We *could* check either check /var/lib/lxc or parse the return of lxc-list to ensure that the container does not already exist -- but this doesn't seem like a show-stopper. Let me know if it is and I can maybe add another option to force a check