Revert "Use kitchen-docker instead of kitchen-vagrant."

This reverts commit 8aa253b5c6db849d014ee5a35b4aa95c416c8a89.
deprecate docker-executor.
Install mesosphere packages from mesosphere apt repo.  Mesosphere's package version consists of two parts, `mesos_version`-`build_version`.  build version is like `1.0.ubuntu1404`.  User can now specify this in `node[:mesos][:mesosphere][:build_version]`.
bump default version to 0.20.0. add marathon+docker configuration to mesosphere sample Vagrantfile.
version 0.2.0
refine readme.