Mesosphere has had an open-source free version for nearly a year now. You can read about it here:
There are three ways to install it: 1) Cloud provider specific templates (e.g. CloudFormation / Azure Resource Manager. GCP support is on the backlog)
2) The ssh installer (this has a UI and a CLI). I believe this doesn't work well for larger installations because of the one to many issue.
3) The advanced install method. This generates a binary that is copied to each node. You can integrate this with Puppet, Chef, Ansible and so on.
https://dcos.io/install/ and https://dcos.io/docs/ has more details on this.