[FEATURE] Support Docker as hypervisor

Hi,

i just implemented support for docker as hypervisor. The docker images should contain sshd, curl, ntpdate and lsb-release.

I tested with this example config:
```
HOSTS:
  test1.former03.de:
    roles:
      - master
    platform: debian-7-amd64
    hypervisor: docker
    docker_image: simonswine/beaker-debian-wheezy
CONFIG:
  log_level: verbose
  type: foss
```

Everything works as expected. 

This is the result of a fast hack. So far it doesn't contain any specs. Feel free to test and give feedback..

cheers,
simonswine