My reading of it left it unclear that -K equated to "prompt for sudo password".
For the first run, we'll need to tell ansible the SSH and Sudo passwords, because one of the thing that the common role does is to configure passwordless sudo, and deploy a SSH key.
This bit makes sense, though you then don't go and configure it to do passwordless sudo, so it's a touch confusing. I did eventually figure it out because you'd supplied two flags to the ansible-playbook command.
In order to use Ansible with SSH passwords (pretty much required for the first run of normal machines (unless you deploy keys with something far lower level, like kickstart), you'll need the sshpass program.
This is probably why I got a touch confused about it. I'd also note that mac users (i.e. Me!) are SOL when it comes to installing sshpass easily (homebrew refuses to package it which means tedium):
custard-tart% brew install sshpass
Error: No available formula for sshpass
We won't add sshpass because it makes it too easy for novice SSH users to
ruin SSH's security.