Versions of Chronos and Marathon to be installed should be parameterized. fix #2
introduced build_version, default is "{{ timestamp}}".
marathon 0.7.0-RC4
ami_description should include versions.
update images to build version 0.20.0.2 (Mesos 0.20.0, Marathon 0.7.0-RC4, Chronos 2.1.0).