Ansible is a config management / orchestration system that doesn't require a central server. It uses SSH as its transport, which makes it relatively easy to use even in small setups.
It has relatively good support for rolling upgrades built in, along with modules for git, apt, docker and other typical installation methods.
A fairly simple use of Ansible would be to replicate trebuchet functionality with the git module. This should already provide us with a good feel for its rolling deploy capabilities.