Add prebuild packages to Dockerfile

This commit fixes #426. Adding needed packages like ntp, curl etc. to
the Dockerfile. This enhances execution speed of specs, because they do
not have to be reinstalled every time the tests are run, when preserving
the docker image.
This is duplicating values defined in `Beaker::HostPrebuiltSteps::DEBIAN_PACKAGES`. `DEBIAN_PACKAGES` can be joined for the same result:

```
dockerfile += <<-EOF
           RUN apt-get update
           RUN apt-get install -y openssh-server openssh-client #{Beaker::HostPrebuiltSteps::DEBIAN_PACKAGES.join(' ')}
EOF
```