Where does CircleCI-2.0 store the ssh private keys?
I still don't get how your config.yml could launch ansible without a step part. Here is my findings, but I'm unsure it is what you're after, but that was too long for a comment. 

According to the documentation here you have to add a add_ssh_keys with the fingerprint of your key as seen in the UI.

- add_ssh_keys:
    fingerprints:
      - "48:a0:87:54:ca:75:32:12:c6:9e:a2:77:a4:7a:08:a4"