So the current MAAS version that loads as the default version under Ubuntu 16.04 is not MAAS version 2.1.3.
2.1.3 does include image support by default for Centos 6.6 and Centos 7. I have tested Centos 6.6, and it works great, just be sure you have your ssh public key loaded first, and login using the user "centos" and your provate key that pairs with the above mentioned public key.
However when I test the Centos 7 image, I found that the Centos 7 network ports are disabled by default on first boot and must be enabled at the console during install (or by script, which is failing in MAAS 2.1.3), and the MAAS deploy scripts may need to be updated to resolve this. There is no obvious way to turn the ethernet ports on, I can't even break in to see what's going wrong in the scripts. So Centos 6.6 is what works now, as of 5/11/17. I have another question open on the Centos 7 issue, so I will update that thread once I get a working Centos 7 running.