Comment 1 for bug 1680175

Problem appears to be a missing minimum version dependency on python3-curtin. This is what I had installed:

$ apt-cache policy python3-curtin
python3-curtin:
  Installed: 0.1.0~bzr425-0ubuntu1~16.04.1
  Candidate: 0.1.0~bzr480-0ubuntu1~ubuntu16.04.1
  Version table:
     0.1.0~bzr480-0ubuntu1~ubuntu16.04.1 500
        500 http://ppa.launchpad.net/maas/next/ubuntu xenial/main amd64 Packages
     0.1.0~bzr470-0ubuntu1~16.04.1 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
 *** 0.1.0~bzr425-0ubuntu1~16.04.1 100
        100 /var/lib/dpkg/status
     0.1.0~bzr365-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages

An 'apt-get install python3-curtin' to 0.1.0~bzr480-0ubuntu1~ubuntu16.04.1 solved the issue and I'm now running maas 2.2.