(amd64)ubuntu@localhost:~$ cat > cfg.yaml <<'EOF'
> config:
>   ubuntu-core:
>     network:
>       ppp:
>         - name: chap-secrets
>           content: mysecret
> EOF
(amd64)ubuntu@localhost:~$ sudo snappy config ubuntu-core cfg.yaml 
open /etc/ppp/chap-secrets: read-only file system