line: accept host key and clone as ansible_user
Created by: jedmund
Follow-up to #18. The SSH URL change got past the credential-prompt hang, but the deploy still fails:
``` TASK [infra_gateway : Pull latest line source] fatal: [nuc-mini]: FAILED! ... "Host key verification failed." ```
Two pieces missing from the git task that `album_sort` already has:
- `accept_hostkey: true` so SSH adds github.com to known_hosts on first contact instead of failing.
- `become: false` so the task runs as the SSH user (`justin`, who has the github SSH key) rather than as root.
After this lands, `make deploy-infra-gateway` should get past the git task and proceed to build + start the line container.